Introduction to Polyurethane Coatings
Introduction to Polyurethane Coatings
Polyurethane coatings are defined as products made from polyisocyanates and polyols. PU coatings have increased since they were first introduced years ago due to:
- Highly versatile chemistry
- Superior properties such as toughness, resistance to abrasion, and chemicals
- Hardness with flexibility
- Adhesion capability to all sorts of substrates
Today, polyurethane coatings are applied using water-borne, solvent-borne, high-solid or powder coating systems.
Chemistry of PU coatings
PU coatings are broadly defined as all the systems based on polyisocyanate. The chemistry of the isocyanate group (-N=C=O) provides the fundamental basis of polyurethane coatings chemistry.
Its high chemical reactivity is coupled with its ability to react with many different chemical partners. This makes the isocyanate group particularly suited for the coatings market.
The isocyanate group can react with any compound having reactive hydrogen. These reactions are explained in the figures below.
Reaction of Isocyanate with Alcohol (R-OH)
Figure 1 shows an isocyanate group reacts with alcohol forming urethane linkages.
Figure 1: Reaction of Isocyanate with Alcohol
Reaction of Isocyanate with Amine (R-NH2)
Figure 2 shows the reaction between an isocyanate and an amine group. The resultant leads to the formation of urea linkages.
Figure 2: Reaction of Isocyanate with Amine
Reaction of Isocyanate with Water (H2O)
Figure 3 shows the reaction between isocyanate with water is known as a "blowing" or "foaming" in polyurethane chemistry. Carbamic acid is the intermediate formed during this unstable reaction. It decomposes to form an amine (-NH2-) and carbon dioxide (CO2). This reaction plays a crucial role in the formation of foamed or cellular polyurethane materials and coatings.
Figure 3: Reaction of Isocyanate with Water
These three reactions are of primary interest giving polyurethanes their unique properties. PU paints may contain urethane, urea, allophanate, and biuret linkages.
There are a wide variety of hydroxyl and/or amine-containing raw materials. They are often called polyols or co-reactants. These are available for reaction with isocyanate-containing raw materials.
Why should you use PU coatings?
Here are some of the top reasons to use polyurethane coatings:
- Durability: Polyurethane coatings display excellent corrosion, chemical, and abrasion resistance properties. These features allow them to withstand harsh conditions that degrade other polymer coatings.
- Flexibility: Polyurethanes form flexible yet tough film coatings. They can tolerate expansion, contraction, vibration, and impact without cracking or peeling.
- Adhesion: Isocyanate chemistry allows polyurethane coatings to bond well to many substrates. It also provides excellent all-around adhesion.
- Aesthetics: PU coatings can be easily formulated in a wide range of colors and high gloss finishes. They retain their visual appeal for longer than many other coating options.
- Customization: You can tweak the base ingredients in your coatings formulation. The raw materials can be tailored to achieve specific hardness, chemical resistance, and UV stability.
The above factors make PUs a versatile protective coating option suitable for many industries. The tunability facilitates matching chemistry to complex performance demands.
PU Technology Classification
PU Technology Classification
When considering PU coatings, you must always remember that there are many options to select from. The selection is based upon:
-
Curing type: One-component and two-component
-
Two-component systems — They consist of polyisocyanate and a polyol or polyamine. These are mixed prior to application and curing by direct cross-linking at room temperature.
- Single-package systems — They are cured when exposed as a film to moisture, oxygen, or heat.
- Composition varieties: Solvent-based and water-based
- Chemistry: Aliphatic and aromatic systems
You can evaluate the attributes of each coating system against your needs to determine the optimal fit. A clear understanding of each system will help you minimize the confusion when selecting the range of PU coatings.
The broad categories of PU technology used in the paint industry are classified as reactive and non-reactive. Here are the major differences between reactive and non-reactive systems.
| Reactive |
Non-reactive
|
- Contain free isocyanate groups (-NCO) that continue reacting after the coating is cured
- Crosslinking density continues to build over months, increasing hardness/chemical resistance
- Typically 1 and 2 component systems
- Reactive with moisture, so humidity can influence curing speed
|
- Lack free, unreacted isocyanates after coating has cured
- Crosslink density remains static after initial cure rather than progressively increasing
- Typically 1 component systems
- More stable curing reaction, not influenced by ambient humidity
|
Reactive PU Systems: Categories and Sub-categories
The reactive polyurethane paints are generally crosslinked. This is due to branched polyols and/or isocyanates or through the formation of allophanate and biuret. There is improved resistance to water, solvents, weathering, and temperature. This is due to crosslinking, whilst increasing hardness and abrasion resistance. But they lead to poorer flexibility if too high a level is being used.
Reactive polyurethane paint systems consist of several categories:
» Two-component polyurethane systems
» One-component polyurethane systems
» Oven-curing or stoving PU systems
» Powder coating systems
They are described in the following section. You can navigate through these links to get detailed information about these categories.
Two-component or 2K PU systems
The two-component family of PU coatings was the first developed. It is characterized by pot life. These systems are also known as two or twin pack or 2K. They are still the most important products.
The primary reaction is between isocyanate and polyols. The reaction starts once the isocyanate and polyol are mixed. This leads to an increase in viscosity and later to gelation. 2K systems include:
- solvent-based formulations,
- water-based formulations, and
- solvent-free formulations.
A significant feature of this family of reactive PU coatings is that curing happens at low temperatures. This produces coatings with excellent properties. Low-temperature curing offers cost and processing benefits, and also the possibility of applying coats on heat-sensitive materials. For example, plastics, and very large items that cannot be heated such as flooring and public transportation.
Another key advantage of the two-component coating is the theoretically infinite storage stability. This is coupled with a rapid curing reaction once the two resins are mixed.
Solvent-based 2K systems
They are quite frequently utilized in the automotive and aviation for refinishing. Here, polyurethane has replaced traditional nitrocellulose and acrylic lacquers. PUs can accept high solids loading and have better properties.
2K solvent-based systems are also used in topcoats for airplanes. Aliphatic isocyanates are mixed with polyester polyols or blends of polyester with acrylic grades.
Water-based 2K systems
The push for low VOC coatings led to the successful launch of water-borne PU coatings. They match closely with solvent-borne systems. This is both in terms of performance and overall appearance. Their high flexibility makes them applicable to polymeric and wooden substrates. These water-based recipes are now commonly used in:
- transportation,
- machinery,
- furniture, and
- protective metal coating applications.
These 2K paints contain dispersible isocyanates and polyols. For example, polyacrylates or polyesters that are emulsifiable or soluble in water. The principal isocyanate is an HDI trimer but IPDI trimers are also being used as is allophanate-modified HDI.
- Aromatic grades are avoided because they react too vividly with water.
- The hydrophobic isocyanate grades can be used as such or can be emulsified by partial reaction with a hydrophilic polyol.
Most water-based paint formulations still need up to 10% solvent to make the polymer form a homogeneous film. The coat is cured at 20°C to 80°C temperature. To ensure adequate polymer network formation, an excess of isocyanate is added. This compensates for the isocyanate consumed during the urea reaction.
Presently available formulations contain no solvent at all. They are predominantly used in the building sector. Here they are applied as sealants or coats for roofing and flooring of car parks and to protect against corrosion. Most of these systems are based on:
- MDI and polyether or oil-modified polyester polyols
- Chain extenders, and
- Catalysts
There is a very high variety of recipes, with pot life ranging from a few minutes to more than an hour, and applicable at temperatures as low as 5°C.
Highly filled 2K systems are used as synthetic mortars for concrete repair. They are also used as a foundation material for heavy equipment. Spray elastomers were also developed for use in belting and mining for pipe linings. They were also used as protective membranes in construction.
Polyurea 2K polyurethane paints normally contain MDI or IPDI. It occasionally has TMXDI with a polyol being a polyoxyalkylene amine or an amine-terminated chain extender. The latter may be diethylene diamine (DETDA) or isophorone diamine (IPDA).
- The polyurea reaction is particularly rapid, hence no catalyst is needed.
- The outlets for polyureas are more diverse than for conventional PU spray coats. This is because of their fast cure and specific characteristics.
- Polyureas end up in paints for pipes, freight ships, industrial floors, transportation liners, and roofs.
- They are attractive in products needing to be put back rapidly in service after painting or where post-cure is impossible or too costly.
One-component or 1K PU Systems
1K moisture-cure PUs are widely used for maintenance and repair. It is capitalized on their easy application and superior mechanical behavior. They are good candidates for painting steel constructions such as bridges, cranes, and primers, or sealers for concrete, and in synthetic mortar. These coatings are:
- prepolymers
- liquid at room temperature, and
- obtained by reacting MDI, HDI, or TDI with polyether or polyester polyols that are linear branched with an isocyanate content below 20%.
These one-component formulations offer the advantage of no metering or mixing need. They have storage stability with a shelf life of up to six months. They perform well in anti-corrosion applications in which corrosion protecting pigments are added. These include:
- Zinc dust
- Zinc-coated polymeric microspheres
- Zinc phosphate
- Lead silicochromate and
- Micaceous iron oxide
Coal tar pitch may be included to make the coat more water-repellent.
Oven-curing or stoving PU systems
Stoving-curing paints are obtained by blending a blocked isocyanate with a polyol. This forms a pseudo-one-component mix that is stable at ambient temperature. When heated to its activation temperature (100-200°C), The isocyanate unblocks and reacts with the polyol forming the coat. Isocyanates may be aromatic or aliphatic. All contain one active hydrogen and a blocking agent which is mostly caprolactam.
Another method of blocking is to form uretidinedione or dimer links. Their use is restricted to applications withstanding heating/cooling cycles. This is because these coatings are all baked at relatively high temperatures. In practice, their main use is for coating metals.
Solvent-based oven-curing coatings
They are used for high-speed paint processes as is the case for continuous coils of steel and aluminum. They are further processed by profilers. They are used in making panels for the construction, appliance, and transport industries.
- Coatings in this category require high flexibility. They are typically applied in a one-machine operation, with TDI. They are commonly used for the base coat, whilst the topcoat contains an IPDI trimer.
- With HDI-based formulations, both base and topcoat have the same isocyanate.
-
Polyols are predominantly polyester grades, acrylics, and phenolics. These are used as long as they resist yellowing.
Water-based oven-curing coatings
Water-borne systems are mainly used for coating appliances and vehicles. They are used as a primer or base layer to provide protection against corrosion and where needed against stone chips. These recipes have dispersions of blocked aromatic isocyanates. They are usually modified with hydrophilic components. This is done for emulsification and polyester polyols or modified epoxies.
The paint is frequently applied through cathodic electrodeposition. This is sometimes referred to as EPD or e-coat. After drying, the isocyanate is activated on stoving.
Powder coating systems
The application of PU powder coatings is relatively small. Due to its lack of need for a solvent and good coating properties, this system generates great interest. These coatings offer:
- durability
- abrasion resistance
- low-temperature flexibility, and
- good aesthetics like all other PU coatings.
They are used in decorative metal coatings for use in automotive, furniture, home appliances, etc.
Operation of Powder Coatings5
The common requirement for PU powder coatings is that all the components must be solid at room temperature. To prevent the PU reaction at any preliminary step, it is important to protect or block isocyanate groups from reacting until they are made available at the oven temperature.
The two methods used for blocking isocyanates are:
- Complexation of the isocyanate groups with a reversible leaving group
- Internal blocking of isocyanate linkage by reversible isocyanate dimer formation to the uretidione
Discover all the reactive polyurethane technologies available in our database.
Non-reactive PU Systems: Main Categories
Non-reactive PU systems contain fully formed polymers with urethane or urea linkages. They do not contain free isocyanates.
Solvent-based lacquers
High molecular weight linear polyurethanes are formed or dissolved in solvents. These PUs are obtained by the reaction of aromatic or aliphatic isocyanates (mainly MDI and IPDI) with:
- polyester or polyether polyols
- chain extenders
The PU polymer has a molecular weight of up to 100,000. Thus, it has a fairly low solids content which is not more than 10%.
The lacquers are brushed or sprayed, and their film is formed by evaporating the solvent. These films are reportedly extremely flexible and elastic on top of being remarkably resistant to mild solvents. Their major outlets are for top coating of flexible substrates such as:
- leather
- fabrics
- shoe soles
- integral skin foam
Another important application is for electronics. Here they are used to coat magnetic tapes. This is done when polyester polyol-based TPUs modified with sulfonate or phosphate groups are used to bind and hold magnetic particles in the film.
Polyurethane dispersions
This family of products grows in significance year after year. PUDs are fully reacted polyurethane systems in the form of small discrete particles. They are a maximum of one-tenth of a micrometer. These particles are dispersed in water to provide a material, both chemically and colloidally stable.
Urethane oils and alkyds
Natural oils such as linseed or soybean can be heated with polyols like glycerol and pentaerythritol.
- In this case, the reaction converts them to diglycerides and monoglycerides.
- These hydroxyl-containing oils can then be reacted with isocyanates (TDI or IPDI). Thus, it forms long-chain hydroxyl-terminated polymers that can be dissolved in solvents.
- They are applied by brush or spray. After solvent evaporation, one obtains a film that crosslinks by air oxidation of the unsaturated oils with the help of catalysts.
Urethane alkyds consist of an alkyd resin in which a portion of the dibasic acid (usually phthalic acid) is replaced by a diisocyanate.
- Ester links are synthesized during the reaction by polycondensation. The isocyanate is added for reaction with the remaining hydroxyl groups to form urethane linkages.
- Urethane alkyds are like traditional alkyds and may be drying or non-drying.
The above-described urethane alkyds should be labeled "urethane-modified alkyds". Urethane oils are often described as "urethane alkyds", "urethane oil", or "uralkyds".
They all exhibit better mechanical and weathering properties than unmodified alkyds. But, they are not nearly as good as the other reactive PU paints. Used in varnishes for flooring and boats, undercoats, and industrial maintenance finishes.
Radiation-curing PUs
This family consists mainly of urethane acrylate coatings. They are one-component, low viscosity, and 100% solid products. They are easy to apply and can be cured by ultraviolet or electron beam energy sources at room temperature.
- Aromatic grades are used in wood, paper, plastic, and ink coats, while
- Aliphatic systems are utilized where non-yellowing is a must. This is the case among others for PVC floor tiles and continuous flooring.
The UV-curable urethane acrylates also end up in adhesives, sealants, and potting or encapsulation compounds.
A prepolymer is obtained from diisocyanate and a polyether or polyester polyol. This reacts with a stoichiometric amount of a hydroxyl-containing acrylate such as hydroxypropyl acrylate. This results in the formation of an oligomer.
- Urethane acrylate oligomers are usually blended with some acrylate monomer. For e.g., glycol diacrylate or trimethylolpropane ethoxylate acrylate as a reactive diluent, and a photoinitiator for UV-curing.
- Benzophenone is a typical photoinitiator that produces free radicals when absorbing UV. It then initiates the crosslinking through the acrylate groups.
EB is powerful enough to eliminate the need for photoinitiators. In EB-curing the electron beams penetrate thick and opaque film layers. Whereas UV-curing is restricted to clear or thin films. Do you want to find out the main difference between UV and EB-curing? Check out the exclusive guide on Radiation-cured coatings.
Discover all the non-reactive polyurethane technologies available in our database.
Classification of PU coatings by ASTM
The American Society for Testing Materials (ASTM) have classified many types of polyurethane coatings.
Type I, one-package pre-reacted, unsaturated aliphatic esters
- Cured by oxidative crosslinking of unsaturated groups and solvent evaporation
- Contains no free isocyanate groups
- Used in architectural coatings and topcoats
Type II, one-package moisture-cured
- Contains free isocyanate groups
- Forms films by the reaction of these isocyanate groups with ambient moisture
- Can use different blocking techniques to preserve isocyanate reactivity
- Used in coatings for leather, concrete, and maintenance
Type III, one-package heat-cured
- Cured by the thermal release of blocking agents and regeneration of active isocyanate groups that react with substances containing active hydrogen groups
- Used in coil and electric wire coatings
Type IV, two-package solvent-borne
- One package contains a prepolymer or adduct having free isocyanate groups and the other contains a catalyst, accelerator, or crosslinking agent such as a monomeric polyol or polyamine and other formulation components
- Acrylic and polyester polyols are common components
- Forms films by combining with a relatively small quantity of components
- Used for plastics, wood, and marine coatings
Type V, two package high solid coatings
- One package contains a prepolymer, adduct, or polyisocyanate, and the other contains a resin
- Forms films by combining with a resin having active hydrogen groups with or without the benefit of a catalyst
- Used for wood, automotive clearcoats and refinishes, and industrial coatings
Type VI, one-package non-reactive low solid solvent-borne
- Characterized by the absence of any significant quantity of free isocyanate or other functional groups
- Referred as lacquer
- Forms high gloss film upon solvent evaporation
- Used in textile coatings
Powder coatings
- Used in automotive coatings, coatings for electrical components, metal surfaces and furniture coatings
Now that you have decided which specific system to select, let’s understand the key ingredients that will ease your selection of formulating PU coatings.
Building Blocks of PU Paints & Coatings
Building Blocks of PU Paints & Coatings
Choosing raw materials is crucial for formulating polyurethane coatings. The right selection makes it possible to customize applications with the desired requirements. Below are some examples of monomers and additives used in preparing PU coatings.
Isocyanates
A PU coating is often characterized by the type of polyisocyanate incorporated into the paint. This can be either aromatic or aliphatic.
| Key Distinctions of Polyisocyanates |
| Aromatic |
Aliphatic |
| Polyurethane coatings based on aromatic diisocyanates tend to yellow in sunlight. Because of this critical difference in light stability, aromatic polyisocyanates are normally used in primer and intermediate layer coatings. |
Aliphatic polyisocyanates are primarily used in outdoor topcoat applications when gloss and color retention are required. Aliphatic isocyanates react more slowly and result into softer coatings than those made of aromatic isocyanates.
|
| Aromatics being cheaper, these isocyanates are used where light stability is not an issue. For example, in primers and in heavily pigmented paints. |
They are used when UV or light stability is a must as is the case in topcoats and many water-based recipes. HDI and adducts are most commonly used, whilst H12MDI is ending up in water-borne systems.
|
|
|
Polyols
The hydroxyl value of polyols lies in the 50-300 bracket. The 3 popular types of polyols are:
- Polyethers
- Polyesters, and
- Acrylics
Acrylic and polyester polyols tend to be preferred for harder coats. These have above-average weatherability. The paint performance is also a function of the branching level. It also depends on the hydroxyl value of the polyol utilized. One has also to select the right mix of amines and solvents.
Polyether polyols
They are made by the reaction of epoxides with compounds having active hydrogen atoms. The reactivity of polyether polyols toward isocyanates is determined by:
- the type of initiator, and
- the epoxide monomer that ends the ring-opening polymerization.
Polyether polyols have low viscosity and high reactivity. This enables polyurethane coatings to become an alternative to 100% solid epoxy-based systems. These are used in coating applications for corrosion protection (DTM).
Polyester polyols
They are made by the polycondensation reaction. This reaction takes place between multifunctional carboxylic acids and polyhydroxyl compounds. Conventional polyester polyols are manufactured by the direct polyesterification of:
- High-purity diacids such as adipic acid, and
- Glycols such as 1,4-BDO
Polyester polyols give polyurethanes better solvent, abrasion, and cut resistance. But, they are usually expensive and more viscous than polyether polyols. Other polyester polyols are obtained from reclaimed raw materials. They are manufactured by transesterification (glycolysis) of recycled polyethylene terephthalate or dimethyl terephthalate. Here distillate bottoms are glycols such as diethylene glycol.
Acrylic polyols
They are a group of amorphous polyols. They have a molecular weight (MW) of 8000-13000 daltons. This is obtained by radical copolymerization of acrylic monomers (ternary or quaternary copolymers). For e.g., acrylic or methacrylic acids and esters.
The source of hydroxyl groups in these acrylic polyols is the utilization in the radical copolymerization reaction. This happens within hydroxyalkyl acrylates or hydroxyalkyl methacrylates as comonomers. The acrylic polyols are used in high-performance polyurethane (PU) coatings.
Also, coatings manufactured using other specialty polyols exhibit superior weatherability. They also have good resistance to chemical and environmental attacks. The raw materials used are:
Polyamines
The amine compounds used in paints are mostly polyoxyalkyleneamines. These are mainly:
- Amine-tipped propylene oxide/ethylene oxide copolymers and
- Amine-terminated chain extenders, such as diethyl toluene diamine (DETDA) or isophorone diamine (IPDA).
Additives for PU Paints & Coatings
Additives for PU Paints & Coatings
Solvents
Solvents are added to lower the viscosity and improve the processing. But they should not react with isocyanates. They should have less than 500 ppm water content when applied in reactive systems. Often, 3 or more solvents are mixed to help dissolve all components of the coating formulation. This enables the formation of a stable emulsion.
Suitable solvents for one- and two-component systems include:
- Esters
- Ketones
-
Ether esters, and
- Polar aromatic or aliphatic types whose boiling point ranges from 50°C to above 150°C.
Solvents that contain reactive groups such as amines should not be used since they react with isocyanate groups. The legislative pressure for reducing solvent content has led to the commercialization of low-viscosity polyols and isocyanates.
Leveling agents
Adding suitable leveling agents can improve the flow properties. Commonly use leveling agents are:
- Cellulose acetate butyrate
- Low molecular weight acrylic resins
- Polyvinyl acetate
- Copolymers of PVC/PVAC, and
- Some urea resins
Silicones, polymeric fluids, and fluorochemical additives can improve flow. This happens by lowering the surface tension of the coating material.
Thickening agents
Suitable thickening agents for polyurethane coating systems are:
- Copolymers of vinyl chloride and vinyl acetate
- Precipitated silicas, and
- Bentonite clay
For increased viscosity, copolymers of vinyl chloride and vinyl acetate can be added to the polyol solution. This is added in quantities of 5-10% based on the solid binder. Precipitated silicas increase viscosity and provide thixotropy. When selecting thickeners and pigment-suspending agents, their compatibility with polyols must be considered.
Pigments and extenders
The following inorganic and organic pigments are suitable for most two-component polyurethanes:
| Inorganic Pigments |
Organic Pigments |
- Titanium dioxide
- Iron oxides
- Nickel and chrome titanates
- Chrome and cadmium yellows
- Cadmium red
- Mixed metal oxide blue
- Chrome oxide green
|
- Phthalocyanine blue
- Phthalocyanine green
- Perylene and quinacridone red
- Monoazo, isoindoline, monoarylide yellow
|
Some organic pigments may catalytically accelerate the curing reaction of polyurethanes. This is due to their metal content. Some organic pigments will not give enough coverage in single-coat applications. This is due to transparency.
There are several conventional extenders for one- and two-component systems. These include barytes, calcium carbonate, talc, kaolin, mica, precipitated and amorphous silica, and various other silicate types.
Flattening agents
The incorporation of conventional flattening agents based on silica allows any desired level of gloss to be obtained. This happens either with clear or pigmented two-component polyurethane coating systems.
Catalysts
Catalysts are generally used in one- and two-component polyurethane coatings to shorten the curing time. Various metal compounds are commonly used catalysts. For e.g., dibutyl tin dilaurate and zinc octoate. They are used in both two-component coatings and one-component moisture-curing types.
References
- The Chemistry of Polyurethane Coatings, Bayer
- Thermoset coatings, F. Aguirre-Vargas, in Thermosets (Second Edition), 2018
- Building Science Series, Issue 7
- Szycher's Handbook of Polyurethanes, First Edition
- Polyurethanes: Science, Technology, Markets, and Trends By Mark F. Sonnenschein