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Fundamentals of Polyester Resins

Most polyester resins used in coating applications are relatively low molecular weight and are amorphous, linear or branched and must be crosslinked to form useful films. As a class, thermosetting polyesters generally provide better metal adhesion and impact resistance than thermosetting acrylics, however TSA’s provide coatings with better resistance to hydrolysis and weathering. The presence of ester linkages in the backbone of polyesters make them more prone to hydrolysis, proper selection of backbone monomers that provide steric hindrance to the ester group linkage (for example NPG provides improved resistance to hydrolysis and weather resistance.

formula for reactant in excess - Fundamentals of Polyester Resins

This article will only consider saturated polyesters which are sometimes referred to as oil free polyesters. Polyester coatings are a large portion of the construction, automotive and aerospace markets as they can be engineered to provide excellent properties including mechanical, impact, UV, and chemical resistance for use in waterborne, high solids low VOC and powder coatings. Linear polyesters account for a large portion of the resins used for coil coating applications. When cured with melamine or blocked isocyanate can provide excellent flexibility, chemical resistance and light stability. Formation of polyesters is accomplished by step-growth polymerization of an alcohol with at least two hydroxy groups and a carboxylic acid with at least two carboxyl groups. Most often polyesters contain a blend of diols, triols and dibasic acid with an excess of polyol to form a hydroxy terminated polyester for reaction with melamine or isocyanate prepolymer to form a coating film. If an excess of dibasic acid is used, the polyester is carboxy terminated for reaction with epoxy, melamine or 2-hydroxyalkylamides. Historically polyester synthesis was referred to as condensation polymerization as the reaction of an alcohol group and a carboxyl group produces water. Other polyester synthesis routes include the reaction of an ester with an alcohol, the reaction of an anhydride and an alcohol and lastly the ring opening polymerization of a lactone. When a diol (DD) reacts with a dibasic acid (CC) in equal molar amounts, the molecular weight builds gradually and is more readily controlled. The reactant in excess will have terminal groups of that reactant. For example:

The average molecule will have terminal hydroxyl groups. Branched polyesters are made from mixtures of monomer that contain one or more monomers which have a functionality F > 2. As the proportion of a monomer with F (functionality) > 2 increases, the Number Average Molecular weight increases and the reaction must be controlled to avoid gelation. A wide range of polyesters are in commercial use, for conventional polyesters cured with melamine or isocyanate prepolymers, the number average molecular weight is in the 2,000 to 6,000 range.

Figure 1 – Increase in molecular weight during polyester synthesis:

graph of the increase in molecular weight - Fundamentals of Polyester Resins

Figure 2 – Common hydroxyl functional monomers are as follow:

formula for common hydroxyl functional monomers - Fundamentals of Polyester Resins

Figure 3 – Common Diacid monomers:

formula for common diacid monomers - Fundamentals of Polyester Resins

Table I – Effect of polyols on Polymer Properties:

table of the effect of polymers - Fundamentals of Polyester Resins
table of the effect of acid - Fundamentals of Polyester Resins

Table II – Effect of acid functional monomers on Polymer Proper­­ties:

As Tables I and II illustrate, proper selection of co reactant monomers can provide a range of performance characteristics to provide an array of performance attributes such as

  • hydrolytic stability (NPG, Sebacic, CHDA)
  • exterior weathering (NPG, BEPD, TMP, TME, HHPA, IPA)
  • hardness ( NPG, TME, TME, CHDM, TA)
  • flexibility (AA, AzA, Seb, CHDA, TA, CDO)

Desired performance can be achieved through the proper selection of a blend of monomers coupled with the selection of the polymer architecture to meet film performance properties.

chart of polymer design considerations - Fundamentals of Polyester Resins

Lastly, the architecture of polyesters can be modified with one or more reactive moieties to form for example urethane, oil, or acrylic modified polyesters.

For additional information concerning polyesters, bio-based resins and raw materials, please navigate to www.ulprospector.com.

Resources:

  • Organic Coatings, Science and Technology, Frank N. Jones et.al., Wiley & Sons, 2017
  • Prospector

Remain Bug Free with Antimicrobial Coatings

Ancient civilizations including those in Egypt, China and India have utilized metals or metal compounds utilizing copper, silver and zinc to combat illnesses caused by microbes, while the ancient Greeks and Egyptians used specific molds and plant extracts to treat infections. Since the arrival of SARS, and more recently COVID 19, there is an increasing awareness and use of antimicrobial materials including antimicrobial coatings to combat the spread of disease-causing microbes. The estimated market value of antimicrobial coatings was over $3.2 Billion USD in 2019 with an estimated adjusted annual growth rate of 10.4% through 2026.

Antimicrobial (AM) agents in the form of paint additives act to either kill microorganisms or to stop their growth. Antimicrobial additives in paints can serve as a paint preservative or as an antimicrobial agent in the cured film. Depending on the choice of antimicrobial additives these materials can function to kill or combat the growth of bacteria, virus, fungus and algae on the coating surface. Control of microbes can be achieved through the use of antimicrobial technologies that keep microorganisms from multiplying or growing, providing hygienic surfaces in hospitals and the food industry and to preserve the integrity of paint films.

This article will focus on antimicrobial additives and approaches to provide antimicrobial functionality in cured films. Applications where AM agents are used in coatings to kill or prevent the growth of the following microbes including:

  • fungi
  • bacteria
  • algae
  • virus

Most biocides used in paints are migratory as they function by releasing the active ingredient to the surface of the coating when exposed to moisture. Longevity of the AM modified paint film depends on the rate of release of the biocide as the concentration of the active ingredient decreases with time.

Depiction of the release of AM agent in paint with time - Learn how to Remain Bug Free with Antimicrobial Coatings

The effectiveness of an AM additive in a cured paint is not only dependent upon concentration, resin system, gloss, PVC, coating surface structure and the environment to which it is exposed.

The use of metals such as silvercopper (and many copper alloys ) and zinc in various forms in paints can be an effective antimicrobial additive. There are several mechanisms by which silver acts as an antimicrobial. One such example is that silver ions react with the thiol group in enzymes leading to cell death. The mechanisms through which copper acts to destroys cells includes the generation of hydrogen peroxide in the cells, excess copper can also bind with proteins resulting in the breakdown of the protein into nonfunctional sections. Zinc pyrithione/2-propynyl butylcarbamate acts both a preservative and as a fungicide. The EPA oversees the regulation of antimicrobial agents and materials and determined that copper alloys kill more than 99.9% of disease-causing bacteria within just two hours when cleaned regularly. Copper and copper alloys are unique classes of solid materials as no other solid touch surfaces have permission in the U.S. to make human health claims. Accordingly, the EPA has granted antimicrobial registration status to 355 different copper alloy compositions.

Metal nanoparticles including PVP and polysaccharide coated silver nanoparticles, MES-coated silver and gold have also demonstrated promise as antiviral agents. Copper nanoparticles have demonstrated antimicrobiological activity with Ecoli, fungus and bacteria.

The use of certain Quaternary Ammonium Silane compounds also provide antimicrobial properties when bonded to a solid surface. Some examples include dimethyloctadecyl (3-trimethoxysilyl propyl) ammonium chloride, alkyldimethylbenzylammonium chloride and didecyldimethylammonium chloride.

More recent literature reveals the impact that surface structure has on antimicrobial properties as a needle like surface structure formed by the bonding of 3-(trihydroxysilyl) propyldimethyloctadecyl ammonium chloride to the surface to destroy microbes by rupturing their outer membrane as they come in contact with surface spikes.

Chemical Vapor Deposited titanium dioxide has photocatalytic activity when exposed to UV light. Its self-cleaning properties are due to its strong oxidizing power that results in anti-bacterial, anti-viral and anti-fungal activity.

Superhydrophobic surfaces are those with a contact angle normally in the range of 150 degrees or greater. The surface structure is characterized by a needlelike micro-structure coupled with components that provide low surface tension. Such surface structures also have efficacy in reducing the ability of microbes to adhere to the surface thus imparting antimicrobial activity.

For additional information concerning the selection of materials to enhance hydrophobicity, please navigate to www.ulprospector.com (EU).

Resources:

  • Organic Coatings, Science and Technology, Frank N. Jones et.al., Wiley & Sons, 2017
  • Prospector
  • PCI Magazine
  • C & EN News
  • Science and Technology of Advanced Materials
  • Wikipedia
  • Global Market Insights

Smart Coatings – The Intelligent Choice

There are many definitions for Smart Coatings, however they all have the common trait of being able to sense and interact with their environment. Smart coatings offer additional functional value to that provided by traditional properties of protection and decoration. A report by Transparency Market Research predicts the global smart coatings market will expand at a compound annual growth rate of 29.8% during the period between 2017 and 2025 and reach 1 billion dollars in sales by 2024.

External stimuli in smart coatings

External stimuli in smart coatings may include properties such as:

  • Anticorrosion
  • Antifingerprinting
  • Antifouling
  • Antimicrobiological
  • Antifungal
  • Color-shifting
  • Easy clean
  • Electrochromic
  • Hydrophobic
  • Hydrophilic
  • Ice-phobic
  • Photovoltaic
  • Piezoelectric
  • Piezo-magnetic
  • Self-healing
  • Solar-reflective
  • Super-hydrophobic
  • Thermochromic

These coating properties can be obtained by the use of novel specialty additives, pigments and/or polymers.

Icephobic coatings either resist the formation of ice on the surface to which ice has poor adhesion or facilitate the release of ice that has formed on the surface. Icephobic coatings have application in the aircraft industry, wind turbines and power lines. There are two types of ice formation that are problematic.

  • Rime ice, more commonly known as frost
  • Glare ice, more commonly called glaze ice, which forms a continuous layer of liquid water which freezes on the surface. Glare ice is particularly dangerous on power lines and aircraft.

An icephobic coating can either be formulated to work for rime ice or glare ice, but not both. For Glare Ice some degree of hydrophobicity is necessary, however the surface structure of many superhydrophobic coatings can actually enhance ice adhesion. The low surface polarity and surface structure of superhydrophobic coatings renders the surface less icephobic than would be expected based on the contact angle. Figure 1 illustrates.

Figure 1 - Learn more about smart coatings

Some studies show that elastomeric polyurethane coatings provide less ice adhesion than that of coatings that are similarly structured but more glassy in nature. The theory is that the surface of the PU elastomeric coating induces slippage between the solid ice and that of the lightly cross-linked PU or silicone elastomeric structure with dangling chains at the surface.

Other approaches utilize freezing point depression on some surfaces or the addition of oils to low surface tension coatings. Lastly, some coatings utilize additives to increase the degree of undercooling required for ice nucleation to form.

Self-Healing Coatings

All coatings are susceptible to scratching and abrasion during their service life. Scratching and abrasion not only has an adverse effect on appearance, but further reduce the effective life expectancy in the event that the coating is applied over an oxidizable metal surface.

Seongpil An, et.al studied self-healing technology based on capsules or fibers. Once the coating is scratched, micro or nano-capsules containing catalyzed liquid polymerizable materials (e.g. drying oils, dicyclopentadiene) are released into the scratch. Figure 2 illustrates Self-Healing technology based on capsules or fibers. Once the capsules are ruptured, polymerization takes place filling the void and functions to reduce moisture ingress and thus improve corrosion resistance as well as the appearance of the coating. Fibers based on thermoplastic poly(e-caprolactone) distributed in an epoxy matrix is one example of self-healing technology to restore film integrity when exposed to heat.

Figure 2- Self Healing Coatings based on Capsules or Fibers

Figure 2 - Learn more about smart coatings

Environmentally sensing coatings

Able to respond to a change in their environment, these coatings have utility for multiple applications. For example some waterborne interior house paints contain a dye that changes color due to exposure to interior light or a change in pH during the drying process. Upon drying, the change in color from for example pink or purple helps to signify sufficient coverage over a similarly colored undercoat.

Coatings that contain a pH sensitive dye and fluorescent molecules are also used to detect corrosion. Another approach is the use of a Rhodamine B-based dopant in epoxy coatings to sense corrosion on both steel and aluminum as it responds to both a decrease in pH and the presence of Fe+++ ions.

Another fast growing area of smart coatings is the use of coatings that are modified to resist colonization of surfaces by viruses or bacteria. Most surfaces contain minute amounts of nutrients such as sugars, oils or phosphorous that serve to enable microbes to grow and reproduce.


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Antimicrobial coatings

Antimicrobial coatings have utility in multiple applications including hospitals, kitchens, public bathrooms, transportation (taxi cabs, Uber vehicles, airplanes) and on hand rails and door knobs. Additives that have been successfully used include materials containing silver in various binders or absorbed onto a porous surface to enable slow release and improve longevity. Quaternary ammonium salts also provide antimicrobial activity, Quaternary ammonium salts can be more effective against viruses and fungi. Copper also provides some antimicrobial activity as well as organic based anti-bacterials such as Triclosan.

Table 1 – Summary of other Smart Coating Applications

Coating TypePrincipalStimulusSmart Response
Solar ReflectiveReflect IR EnergyLight colors and dark colors using doped mixed metal oxidesSunshineProvides cooler surface, saves air conditioning cost
PiezoelectricPigment generates electrical current when stressed(Pb-Zr-Titanate)VibrationCreates an voltage when subjected to mechanical stress
PiezomagneticPolycrystalline materials generate magnetic field when stressedVibrationCreates a magnetic field when subjected to mechanical stress
ThermochromicChange color in response to temperature liquid crystals and Leuco dyeTemperatureIndicates temperature change in a designated range
ElectrochromicPolymeric electrolyte that changes color when exposed to an electric currentElectric currentColor change, aesthetic appeal, indicator
Hydrophobic/hydrophilicSurface modification coupled with adjusting surface tensionMoistureAdjust water contact angle to repel (hydrophobic) or attract moisture (hydrophilic)

For additional information concerning the selection of materials to enhance hydrophobicity, please navigate to www.ulprospector.com (EU).

  • Organic Coatings, Science and Technology, Frank N. Jones et.al., Wiley & Sons, 2017
  • PCI Magazine
  • Science Direct
  • Shape Memory Assisted Self- Healing Coatings, 2013, Material Science, Luo and Mather
  • Transparency Market Research: Smart Coatings Market – Global Industry Analysis, Size, Share, Growth, Trends, and Forecast 2017-2025
  • Seongpil An, Min Wook Lee, Alexander L. Yarin, Sam S. Yoon, A review on corrosion-protective extrinsic self-healing: Comparison of microcapsule-based systems and those based on core-shell vascular networks, Chemical Engineering Journal, Volume 344, 2018, Pages 206-220, ISSN 1385-8947, http://doi.org/10.1016/j.cej.2018.03.040.

The views, opinions and technical analyses presented here are those of the author or advertiser, and are not necessarily those of UL’s Prospector.com or UL LLC. All content is subject to copyright and may not be reproduced without prior authorization from UL or the advertiser. While the editors of this site may verify the accuracy of its content from time to time, we assume no responsibility for errors made by the author, editorial staff or any other contributor.

A Guide to Providing Perfect Coating Adhesion

Paint films for nearly all aesthetic and functional applications above all else must provide adhesion to the desired substrate. Accordingly, one must take into account multiple considerations when formulating a coating that provides acceptable adhesion for the intended application. Critical considerations and how they impact adhesion include:

  1. Surface wetting
  2. Mechanical effects and internal stress
  3. Surface chemistry and bond strength
  4. Pigmentation
  5. Evaluation of adhesion

1. Surface wetting – The relationship between surface wetting and adhesion is the first factor to be considered in designing a coating to optimize adhesion. If a coating in a liquid state does not spread spontaneously over the substrate surface, then there is limited opportunity to form mechanical and chemical bonds with the substrate surface.

A liquid will spread spontaneously on the surface of a material if the surface tension (force/unit length or dyne/cm) of the liquid is lower than the surface free energy of the solid to be coated. For example, the image below provides a visualization of various degrees of wetting properties for a drop of liquid applied onto the surface to be wet.

Figure 1 – Images of Various Degrees of Substrate Wetting

Image of degrees of substrate wetting - A Guide to Providing Perfect Coating Adhesion

Accordingly, in Table 1, when the Liquid Surface Tension (LST) is lower than that of  the Solid Surface Tension (SST), then wetting of the solid will occur. The greater this difference, the greater the opportunity the liquid has to wet and spread on the surface of the solid. Waterborne paints have a more difficult time spreading on surfaces due to the relatively high surface tension of water in comparison to most organic solvents.

Accordingly, to improve wetting of waterborne coatings, organic cosolvents and appropriate wetting agents are normally employed. In summary, when LST < SST, wetting occurs.

Table 1 – Liquid Surface Tension (LST) and Solid Critical Surface tension (SST) (dynes/cm) @ 20° C

Table of surface tensions - A Guide to Providing Perfect Coating Adhesion

2. Mechanical adhesion and internal stress – The profile of the substrate the coating is to be applied to also can affect adhesion. Smoother surfaces are more difficult for coating adhesion as the surface area is lower and provides less area for the coating to interlock with the substrate. However, if a coating is extremely rough, it can be difficult for a liquid coating to wet and penetrate surface crevices. This is illustrated in the diagrams listed below in Figure 2.

Figure 2 Surface interactions between a coating and substrate

Image of surface interactions - A Guide to Providing Perfect Coating Adhesion

The microscopic surface profile in sketch B will provide better adhesion than that in sketch A as the coating provides greater opportunity to interlock with the substrate. Surface C has pockets and pores that are not easily penetrated by the coating, resulting in air pockets that can trap moisture and soluble ions resulting in blisters and corrosion (if substrate is an oxidizable metal) and thus poor long-term adhesion and eventual film failure.

In summary, from a mechanical adhesion standpoint, liquid coatings with low surface tension and low viscosity help promote better wetting and microscopic penetration (capillary action). Adhesion can also be adversely affected by stresses that occur as a result of shrinkage as a coating dries or cures. Environmental effects over time such as exposure to moisture, light, heat, pollutants and thermocycling also play an eventual role to degrade adhesion.

3. Surface chemistry and bond strength  In addition to surface tension and surface profile of the substrate, available substrate functional groups may provide sites for covalent and hydrogen bonding to the coating components to further enhance the adhesive bond strength to the substrate.

Table 2 – Adhesive bonding forces

Table of adhesive bonding forces - A Guide to Providing Perfect Coating Adhesion

As Table 2 illustrates, the highest bond strength to the surface is provided by covalent bonds, such as those provided for example the reaction of a dual functional trialkoxy silane coupling agent between the coating and the metal surface.

Most metal surfaces are supplied with a thin layer of oil to slow the rate of oxidation. The oil also lowers the surface energy and thus is more difficult to wet. For this reason, metal surfaces -for example steel, zinc coated steel and aluminum- are normally cleaned prior to painting to remove oils and then pretreated to form, for example, a zinc phosphate or iron phosphate treated surface. The phosphate groups serve to enhance adhesion of the coating through hydrogen bonding of the metal surface to reactive sites on the polymer.

Figure 3 Example of Hydrogen bonding to a metal surface pretreated with Zn.Phosphate

Formula of hydrogen bonding - A Guide to Providing Perfect Coating Adhesion

Reactive groups on the polymer back bone or through the addition of a di or multifunctional adhesion promoter containing epoxy, amino or silane functional coupling groups can further react with a suitable pretreated metal surface to form covalent bonds that provide added adhesive strength between the metal and the coating.

For glass or silica rich surfaces, coupling agents such as amino silanes can also serve to enhance adhesion by reacting with a resin backbone containing an epoxy group with the alkoxy functional silane portion of the coupling agent bonding to the silica surface to form a siloxane.

Plastics are more difficult to wet as they have a lower surface free energy that may be further lowered by the presence of mold release agents. Adhesion to polyolefins can be improved by increasing their surface free energy through UV irradiation, once a photosensitizer is applied, or flame treatment that generates hydroxyl, carboxyl and ketone groups.

These functional groups on the plastic surface provide higher surface energy to improve wetting as well as hydrogen bonding sites for polymer functional groups on the coating. Other ways to improve adhesion to thermoplastics is to include an appropriate solvent in the paint to solubilize the plastic surface and enable intermixing of the coating at the plastic-coating interface.

4. Pigmentation – The level and type of pigment used in a primer not only affects coating substrate adhesion, but also how long it will adhere to the surface. Most primers are formulated at or slightly below Critical Pigment Volume Concentration (CPVC) to maximize topcoat adhesion (rougher primer surface and higher free energy) as well as many other coating properties (Figure 4).

Table of the Effect of CPVC on coating properties - A Guide to Providing Perfect Coating Adhesion

The use of more polar pigments may provide ease of wetting during the pigment dispersion process, but may degrade long term adhesion as they are more susceptible to moisture migration and disbondment at the coating-substrate interface. Plate like pigments and pigments that have very low or no water-soluble components also enhance longevity.

Method A and B - A Guide to Providing Perfect Coating Adhesion

5. Evaluation of adhesion There are multiple ways to determine and quantify the adhesion of organic coatings to a substrate. Two of the most common means of determining adhesion include ASTM D3359 (Cross Hatch Tape Adhesion) and ASTM D4541 (Pull-Off Adhesion). ASTM D3359 describes two methods to determine cross hatch tape adhesion: method A is a simple X, where method B is a lattice pattern. Method A is used in the field and for films > 5 mils, whereas Method B is used for lab determinations. Ratings are as illustrated below:

Classifications are by area of the cross hatch removed by specialized adhesion tape and include:

5B (no area removed) > 4B (less than 5%) > 3B (5 – 15%) > 2B (15 – 35%),1B (35 – 65%) and 0B (greater than 65%)

ASTM D4541 (Pull-Off Adhesionutilizes a device to measure the Pull Off Strength of a dolly glued to the surface of the coating. The device determines the force required to disbond the coating in pounds per square inch. This not only quantifies the amount of force required to pull off the coating, but also the type of failure (cohesive or adhesive), how and at which layer the coating fails (topcoat to primer, primer to substrate etc.).

PosiTest AT-A - A Guide to Providing Perfect Coating Adhesion
PosiTest AT-A Automatic Adhesion Tester (SOURCE: DeFelsko)

Sources:

Better Performance through Rheology

The difference between a paint with trouble-free performance and failure can depend in large part on the rheology of the paint. Rheology is defined as the science of flow and deformation and influences properties such as:

  • Transfer of resin and paint
  • Pigment dispersion
  • Application (brush, roller, reverse or direct roll coat, spray, disc and flow coat)
  • Film formation (flow, leveling and film coalescence)
  • Storage stability (resistance to hard settling of pigment)

In contrast, viscosity can be defined as the resistance to flow. A discussion of flow and leveling is meaningless without consideration and understanding of viscosity. Simply stated, viscosity is the resistance of a liquid to flow and can be defined in measurable values. Viscosity is expressed as the relationship between shear stress and shear rate.

                       ϒ (shear stress) = F (force) / A (area)

                       D (shear rate) = V (velocity) / C (thickness)

Shear rate is expressed as sec-and shear stress as dyne/cm2
Accordingly, viscosity can be expressed as shear stress / shear rate:

                       η (viscosity)  = ϒ (shear stress) / D (shear rate)

It follows that the units of viscosity are dyne/cm/ sec-1 or dyne-sec / cmor poise. Fluids are classified as:

  • Newtonian (linear relationship of viscosity with shear rate)
  • Thixotropic or pseudoplastic (decrease in viscosity with increasing shear rate)
  • Dilatent (increase in viscosity with increasing shear rate)

Table 1. Viscosity units of measurement

Viscosity units of measurement - Learn more about the rheology of paints

Figure 1. Single point viscosity measurement v. multi-point viscosity measurement

Graph of single point viscosity - Learn more about the rheology of paints

As Figure 1 indicates, a single point viscosity measurement does not provide the information necessary to determine if a paint is Newtonian, dilatant or thixotropic. Accordingly, to properly formulate a paint for various paint processes, it is necessary to know the viscosity characteristics over a range of shear rates. Multi-point viscosity determinations and rheology adjustments enable optimized pigment dispersion, resin and paint transfer, application, paint flow, leveling and storage stability.

Viscosity characteristics of various fluids

image explaining the viscosity of various fluids - Learn more about the rheology of paints

Figure 2. Viscosity and shear rate requirements for various paint processes

Graph indicating viscosity and shear rate - Learn more about the rheology of paints

As Figures 3 and 4 illustrate viscosity requirements for coating processes such as resin and paint transfer, pigment dispersion, application, film formation and storage stability are dependent on rheology. For example, in high speed pigment dispersion and application properties, a degree of thixotropy (shear thinning) aids processing, sag resistance and settling resistance.

Fig. 3 Type of viscosity determinations for various processes

types of viscosity - Learn more about the rheology of paints

Fig. 4 Rheology profile for multiple paint processes

chart of rheology profiles - Learn more about the rheology of paints

Fig. 5 Viscosity requirements for mill base formulation

Millbase formulation - Learn more about the rheology of paints

Multiple rheology/control modifiers can be found using the Prospector Search Engine and are available to modify waterborne and solvent-borne paints to adjust application properties as well as for resistance to hard setting. There are multiple ingredients and variables that influence rheology in a coating formulation.

The issues that impact rheology in paints include:

  • Coating ingredients
    • Binders (solution versus latex or dispersion)
    • Pigments
    • Filler pigments and extenders
    • Pigment dispersants
    • Surfactants
    • Amines amount and type (waterborne paints)
    • pH (waterborne)
    • Cosolvent
  • Customization of rheological properties
    • Criteria for rheology modification and selection
    • Flow and leveling agents
    • Surfactants
    • Other additives

The viscosity of latex paints tends to exhibit excessive shear thinning behavior and is dependent on multiple compositional factors as listed above. For latex paints, when the viscosity at high shear rates is adjusted for proper application, the viscosity at low shear rates for proper leveling tends to be high. This is the reason why the leveling of latex paints tends to be poorer than that of solvent-borne paints. This is most pronounced at higher gloss levels. Accordingly, to counteract this phenomena, associative thickeners are used. In simple terminology, associative thickeners can be defined as a water-soluble polymer containing multiple hydrophobic groups.

Some common thixatropes and their incorporation include:

  • Organo clay – Added during pigment dispersion step
  • Hydrogenated castor wax – Added to mill base while cooling/heat activated
  • Polyamide – Added to mill base while cooling/heat activated or can be preactivated and added during letdown
  • Fumed silica – Added during letdown

Rheology control agents for waterborne coatings include:

  • Cellulosics
    • Hydroxyethyl cellulose
    • Carboxyl functional cellulose
    • Methyl cellulose
  • Polyamides
  • Synthetic clay
  • Colloidal silica

Associative thickeners types for waterborne coatings include:

  • HEUR (Hydrophobically Modified Ethoxylated Urethanes)
  • HASE (Hydrophobically-Modified Alkali-Swellable Emulsions)
  • HMEC (Hydrophobically-Modified Hydroxy Ethyl Cellulose)
  • HEURASE – Hydrophobically Modified Ethoxylated Urethane Alkali Swellable Emulsion)

Fig. 6 ASTM D2801 Sag Resistance- Images of applied paint before (left photo) and after (right photo) the addition of a rheology modifier

sag resistance - Learn more about the rheology of paints

Figure 6. Illustrates the difference in vertical sag resistance of the same paint with (right photo) paint properly adjusted with a thixatrope compared to the photo on the left prior to modificationIn summary rheology plays a major role in providing a paint that offers ease of pigment dispersion, good fluid transfer, acceptable application properties and long term resistance to hard settling. Additional information concerning rheological materials can be found using Prospector’s search engine for key words such as rheology, thixotropy, flow and thickener.

Resources

Prospector Knowledge Center and Search Engine

Wikepedia

Organic Coatings, Science and Technology, Third Edition, Wiley, Wicks e.al. 2007

Organic Coatings, Science and Technology, Third Edition, Wiley, Jones e.al. 2017

www.warnerblank.com

The Rapidly Growing Segment of Direct to Metal Coatings

Direct to Metal Coatings (DTM) is a rapidly growing segment of the coatings industry. This growth is related to cost reduction attributed to improved efficiency, time savings and fewer production steps. These coatings are used in the heavy construction industry, building products and product finishing. Many of these applications require performance in demanding exposure conditions such as oil drilling, off shore oil rigs and foundries. The compound annual growth rate of DTM coatings is estimated to be about 10%. DTM coatings are applied by spray, brush, roll and coil coating. Substrates include aluminum, cold rolled steel, hot rolled steel and coated metals (e.g. hot dip galvanized steel, galfan, galvalume, electrogalvanized steel and plated metals).

By definition, DTM coatings are applied directly to a metal surface with the ability to adhere without the need for extensive cleaning or pretreatment. Ideally these coatings can be applied in one step directly to the metal. However, DTM coatings can also be comprised of one coat of primer and one coat of topcoat applied over metal surfaces that are properly prepared to eliminate surface contaminants and oxides. The primary advantage of DTM coatings is that they do not require a multistep operation of cleaning, pretreatment and sealing prior to painting. Current DTM technologies include solvent borne, waterborne and high solids. They can be one- or two-component acrylic, epoxy or polyurethane, or comprised of unsaturated polymers/oligomers that cure through polymerization.

Image of substrate wetting - Learn more about Direct to Metal Coatings

There are multiple issues to consider in designing a DTM coating that provides longer term performance. These include:

  • Wetting of the substrate
  • Initial adhesion
  • Longer term adhesion and corrosion resistance

Wetting of the substrate

Wetting of the metal surface is a major factor that effects initial adhesion. If the coating does not readily spread or wet the surface, adhesion will be adversely effected. Stating this in a another way–the surface tension of the substrate must be higher than that of the applied coating to ensure good flow and leveling. In the diagram above, the blue sphere represents a paint droplet, and the yellow line represents a metal surface. The droplet on the right completely wets the metal surface thus providing the best opportunity to provide adhesion.

There are two ways to ensure good substrate wetting. From a substrate standpoint, the first is to increase the surface area of the substrate–for example, through abrasion and/or sandblasting. This process also removes the metal oxide and hydroxide layer to provide a surface more amenable to forming a longer lasting surface bond. The second way is to modify the coating to ensure good wetting (e.g. lower surface tension) through the addition of suitable wetting agents as well as solvents or co-solvents which may depress the surface tension.

Once adequate initial wetting is achieved, the second consideration is reviewing the factors that contribute to initial metal adhesion.


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Initial adhesion

Initial adhesion may be defined as the quality of adhesion to the substrate surface after the paint is cured, but prior to exposure to natural weathering and/or accelerated testing. Initial adhesion of the cured film can be quantified by such tests as ASTM D3359 Cross Hatch Tape Adhesion and/or ASTM D 4541 Pull-off Strength of Coatings that quantifies adhesion in pounds per square inch. Some considerations to enhance initial adhesion after volatiles have vaporized from the paint film include:

  • Resin systems with functional groups that promote bonding to the metal surface
  • The presence of suitable adhesion promoters and coupling agents
  • The number and type of crosslinks

Resin systems with functional groups

Resin and crosslinker systems with the ability to form hydrogen bonds or covalent bonds with the layer of oxide and hydroxide on the metal surface normally provide the best initial adhesion. Long-term adhesion and corrosion protection depends on the resin backbone and crosslinking.

Metal Substrate - - Learn more about Direct to Metal Coatings

The presence of suitable adhesion promoters and coupling agents

To promote adhesion, resins and crosslinkers that contain a plethora of active hydrogen donor and accepting groups should be used. Such resins contain one or more of the following functional groups:

  • carboxyl (hydrogen donating group)
  • amine (hydrogen accepting group)
  • hydroxyl
  • amide
  • urethane
  • phosphate (all hydrogen accepting or donating)

The number and type of crosslinks

Accordingly it makes sense why epoxies crosslinked with amino-amide groups (hydroxy, ether, amino and amide functional groups), polyurethanes and polyureas (from moisture cure urethanes for example) provide excellent adhesion to metal surfaces. Thus, they are used widely in direct to metal applications.

The addition of a suitable silane coupling agent also can enhance both initial and long-term adhesion properties. A coupling agent is a molecule that is comprised of a reactive group on one end of the molecule ( Y ) for reacting with a functional group on the polymer chain with the other end of the coupling agent ( – Si – OR3 ) reacting with the metal surface.

Formula 2 - - Learn more about Direct to Metal Coatings

In the above molecule, the -OR groups attached to silicon can be methoxy or ethoxy, where the Y portion of the molecule is a functional group such as amino, epoxy, isocyanate, methacrylate or vinyl. The reaction involves first hydrolysis of the alkoxy group to form a silanol which undergoes a further reaction with the hydroxyl groups on the metal surface. The other end, or Y portion, of the coupling agent reacts with a functional group on the resin backbone.

Formula 2 - Learn more about Direct to Metal Coatings

Table I- Examples of trialkoxy organofunctionalsilanes and their application

R = Reactive Group onR-Si (-OCH3) or R-Si (-OCH2CH3)R group Reacts withReactive SilaneExampleTrialkoxy Silane ReactionApplication
AminoEpoxy functionality 3-aminopropyl-triethoxysilaneWith –OH on surface as well as self-crosslink to form– Si – O – Si –Coatings for glass as well as oxides of Al, Zr, Sn, Ti and Ni
EpoxyAmino functionality3-glycidyloxypropyl trimethoxysilaneWith –OH on surface as well as self-crosslink to form– Si – O – Si –Coatings for glass as well as oxides of Al, Zr, Sn, Ti and Ni
Meth–acrylateAcrylic resin polymerization3-methacryloxypropyltrimethoxysilaneSelf-crosslink with another silane to form– Si- O – Si – and with –OH on the surfaceMoisture cure resins with improved adhesion, physical and environmental performance
N/AN/AN-octyltriethoxysilaneForms– Si – O – Si –Water repellency, improved hydrophobicity
VinylVinyl or acrylic resin polymerizationVinyl-trimethoxysilaneForms– Si – O – Si –Moisture cure resins with improved adhesion and film integrity. Also used as a moisture scavenger
IsocyanateHydroxyl, Amino or Mercapto3-isocyanatopropyl-triethoxysilaneWith –OH on surface as well as self-crosslink to form– Si – O – SiCoatings for metallic and inorganic oxides, also moisture cures
SilaneSIVO Sol-GelVOC Free Waterborne Surface Treatment for various metals and surfaces

Longer term adhesion and corrosion resistance

Lastly, to provide longer term adhesion and corrosion protection, the DTM primer should be formulated with a quality resin system, contain corrosion inhibitive pigment(s) and resist moisture penetration. The latter quality can be accomplished by increasing hydrophobicity and crosslink density. A long lasting moisture resistant primer also has the ability to resist hydrolysis of the cured film.

Figure 2 illustrates the type of corrosion protection that can be achieved with a formulation that provides excellent substrate wetting, superb initial adhesion, long-term corrosion resistance and high hydrophobicity.

Figure 2. Rust Armour primer with a two component urethane topcoat formulated by Chemical Dynamics–utilizing a high crosslinking resin system with and without combinations of hydrophobic pigment modification (SNTS).

10,000 ASTM B117 Salt Spray of Properly Formulated Direct to Metal 2 Coat Paint System (bottom row represents paint film removed).

Image of topcoats - learn about Direct to Metal Coatings

Long term corrosion resistance is an important consideration along with the selection of a resin/coating system that provides wet adhesion and minimizes the penetration of moisture and oxygen. As resin Tg and cross-link density increases, moisture and oxygen penetration decreases. In addition, low permeability rates help to provide wet adhesion as less water will desorb when the coating is removed from its service environment. Resins with a high amount of aromatic character (bisphenol A based epoxies, polycarbonate and styrenated resins) have low oxygen permeability. Halogenated resins such as vinyl chloride, copolymers, chlorinated rubber and fluorinated polymers such as poly (vinylidene fluoride) all have low water solubility and thus low moisture permeability rates1 (see Table II).

In summary, the formulation of DTM coatings to deter corrosion is a complex undertaking and depends on the metal substrate, service environment, pigment level and type of resin selection. For additional information concerning  resin and material selection to formulate corrosion inhibitive coatings, please navigate to www.ulprospector.com.

Sources:

  1. www.faybutler.com/pdf_files/HowHoseMaterialsAffectGas3, Welding Journal.

References:

Prospector Knowledge Center and Search Engine

Zeno W. Wicks Jr., Frank N. Jones, Socrates Peter Pappas, Douglas A. Wicks. (2007). Organic Coatings: Science and Technology, Third Edition.

Wiley, Jones e.al. (2017) Organic Coatings, Science and Technology, Fourth Edition.

Nanoscale Protection For High-Performance

ORIGINALLY POSTED IN THE EUROPEAN COATINGS JOURNEY 07/08/2019

A new generation corrosion control coating technology with high crosslink density. By Atman Fozdar, Ronald Lewar- chik, Raviteja Kommineni, bat365在线平台, USA.

Figure 1: Schematic representation of mechanism by which RA Exp1 penetrates rust and bonds with base metal.

An innovative technology that offers improved performance, saves material and labour costs and eliminates the need for an epoxy primer coat. A single component polymeric penetrant reacts with the corroded base metal to form a long- lasting bond and increase the structure’s useful service life. This coating technology has far-reaching potential, for example in off-shore applications, chemical processing and automotive re- finishes.

Mild steel is one of the most used alloys for different kinds of applications be- cause of its low cost, abundant supply and easy fabrication. But corrosion of steel is one of the major issues faced by transport (e.g. automobiles, aircraft, ships) and infrastructure (e.g. pipelines, buildings, bridges, oil rigs, refineries) industry which directly affects its structural integrity, resulting in issues related to safety and maintenance of steel structures. According to the research published by NACE International [2], corrosion is responsible for losses over $ 2.5 trillion every year. There are different methods to counter corrosion such as, using corrosion inhibitive lining, electroplating, organic polymeric coating and chemical vapor deposition. Ap- plying protective organic coatings to metallic substrate, especially aluminium and steel, is an effective way to protect those substrates against severe corrosive environments. Organic coatings can minimise corrosion of metallic substrates by three main mechanisms: barrier, sacrificial and inhibition.

We often see early signs of corrosion on a steel structure for a variety of reasons. It may be caused by poor surface preparation or application of protective coatings or possibly environmental factors such as acid rain, high humidity, temperature variations, condensation of moisture, chemical fumes, and dissolved gases in case of structures submerged in water or soil. Among the factors listed above, improper surface preparation is one of the most important factors that contributes to the corrosion of steel structures and can lead to loss of structural integrity and structure before the end of its useful service life. If there is a way to protect the structures after observing initial signs of corrosion, without going through labour-in- tensive tasks such as coating removal, clean- ing, pre-treatment and recoating application, then this can significantly increase its service life, more efficiently and economically.

Table 1: Comparison of physical and chemical properties of RA Exp1 with other systems.

Results At A Glance

  • We have developed a single component polymeric penetrant that can be applied with or without surface preparation over clean or lightly corroded steel/aluminium.
  • The coating contains nanosized reactive materials which first penetrate the rust and then migrate to the non- corroded metal surface, polymerising to form a highly crosslinked and protective network.
  • Results over cleaned pre-treated steel surfaces can exceed 10,000-hour salt spray with no blisters or scribe creep when top coated.
  • The new innovative technology offers improved performance, eliminates the need for an epoxy primer coat, and saves labour and material costs.

Experimental

One unique aspect of low molecular weight oligomers used in RA Exp1, is a prevalence of three types of reactive unsaturation on the resin backbone and low molecular weight reactive diluents. The three types of double bonds offer a synergistic curing mechanism that results in ancillary curing properties and high crosslink density that inhibits the penetration of soluble salts and moisture. Corrosion resistance is further improved when this resin blend is coupled with corrosion inhibitor pigments such as organically modi- fied zinc aluminium molybdenum orthophosphate hydrate and zinc-5-nitroisophthalate and unique conductive particles. Graphical representation of how RA Exp1 penetrates rust is shown in Figure 1. After penetrating the surface of the substrate, low molecular weight unsaturated monomers and oligomers, chemically bond/crosslink with other reactive sites, forming a highly crosslinked network which is impermeable to moisture and other soluble salts responsible for aggravating corrosion.

Hydrophobic and superhydrophobic variations of RA Exp1 were produced by adding superhydrophobic nano-textured silica [3]. This additive is naturally superhydrophobic having both hydrophilic/phobic sites and produces a volumetric hydrophobic coating. Hence, even if the surface of the cured coat- ing is abraded due to normal wear and tear experienced in the field, the underlying layers will still repel moisture. We formulated a separate design of experiments for RA Exp1 (with and without the additive) and 2-component polyurethane topcoat (with and without the additive).

Protection Demonstrated In Salt Spray Testing

Variations of RA Exp1 with and without the additive were applied on zinc nickel treated cold rolled steel substrate, which was later top coated with a 2k polyurethane coating with and without the additive at 125 μm dry film thickness (DFT) each. A salt spray test was performed in a salt spray cabinet in accordance with the ASTM B117 standard, after which all the panels were cured at ambient temperature for 7 days. Coated panels with an artificial defect (scratch with a dimension of 106 mm x 2 mm, created using a 1 mm scribe tool) were used to accelerate the corrosion process. All coated panels were placed in a test chamber at an angle of 45 ° and ex- posed to the 5.0 wt.% NaCl solution at 40 °C. The condensate collection rate and relative humidity were at least 1.0 to 2.0 ml/h per 80 cm2 (horizontal collection area) and 95 %, respectively. The protective performance of the coating was further investigated with the emphasis on size and distribution of corroded or damaged area on the coated sample surfaces after 10,000 hours of salt spray exposure.

Figure 2 shows 10,000-hour salt spray expo- sure, three of the four systems with RA Exp 1 as the primer and a 2K polyurethane topcoat show no scribe or face blister and/or corrosion. The top four photos show different systems after 10,000 hours of salt spray expo- sure and the bottom four photos show the extent of corrosion underneath the coating (of the same systems) after removing bottom half of coating using paint stripper.

Low Impedance Due To Conductive Nanoparticles

The barrier protection properties of RA Exp1 was investigated by performing EIS on Zinc phosphate pre-treated cold rolled steel, the results of which were compared with those of commercially available coatings based on conventional 2-component epoxy and moisture-cured urethane system. A three- electrode paint test cell (reference electrode: saturated Calomel electrode (SCE), counter electrode: working electrode: steel samples in 14.6 cm2 area) was used to perform the EIS measurements [1]. Impedance quantifications were made at open circuit potential (OCP) which were maintained potentiostatically in the frequency range of 0.1 to 100 KHz and at amplitude sinusoidal voltage of ± 60 mV. The four samples (RA Exp1, 2k epoxy and two moisture-cured urethane samples) were immersed in 40 mL NaCl solution (3.5 wt.%) and EIS measurements were per- formed over a period of 40 days.

Initial Bode and Nyquist plots (Figure 3a & 3b respectively) indicate that all coating variations show a capacitive behaviour with high impedance values. RA Exp1 was found to have relatively lower impedance values compared with other control samples, which could be attributed to the conductive/anti-static nature of the coating due to the addition of conductive nanoparticles and additives to enhance corrosion resistance.

Figure 2: ASTM B117, 10,000 hour salt spray exposure.
Figure 3a: Bode plot of RA Exp1, 2K Epoxy, Moisture cured urethane 1 & 2 (Initial). Figure 3b: Nyquist plot of RA Exp1, 2K Epoxy, Moisture cured urethane 1 & 2 (Initial).
Figure4a: Bode plot of RA Exp1 , 2K Epoxy, Moisture cured urethane 1 & 2, after 50 days (1,000hours) of exposure.
Figure 4b: Nyquist plot of RA Exp1, 2K Epoxy, Moisture cured urethane 1 & 2, after 50 days (1,000 hours) of exposure.

Greater Resistance To Electrolyte Diffusion

Figure 6 shows a simplified equivalent circuit for a metal substrate protected by a semi-permeable coating layer, ignoring the coating resistance of negligible magnitude. The values of circuit elements in equivalent circuit networks can be used to directly characterise coating performance. Pore resistance (Rp) values extracted by fitting equivalent circuit model as a function of exposure time can be used to compare the performance and rank various coating systems. Figure 5 shows a plotted graph containing the logarithm of pore resistance (RP) vs. exposure time (hours), which indicates that Rp of 2K epoxy decreases with time whereas, RA Exp1, moisture-cured ure- thane 1 and urethane 2 are nearly constant for 1,000 hours of exposure to a 3.5 % NaCl solution.

After 1,000 hours of immersion time, impedance values of moisture-cured urethane samples 1 & 2 decreased significantly while RA Exp1 and 2K Epoxy were able to maintain their impedance values without showing a significant decrease. As shown in Figure 4a & 4b, the behaviour of moisture-cured urethane 2 changed from 1 to 2 constant. This could be due to the diffusion of electrolyte to coating and substrate interface; hence, a double layer could be formed below coating layer. For other samples including RA Exp1, no such behaviour was observed which suggests the coating layer was more resistant to the diffusion of electrolyte and soluble salts.

RA Exp1, 2K Epoxy and various moisture-cured urethane systems were spray applied on clean zinc phosphate pre-treated cold rolled steel and sanded cold rolled steel panel at 125 μm dry film thickness (DFT) and were allowed to cure at ambient temperature for a period of 7 days before characterising the physical and mechanical properties. Table 1 provides a com- parison of the physical and chemical properties of the new technology with other systems.

Figure 5: Log of Pore Resistance (Rp) Vs. exposure time.
Figure 6: Equivalent circuit diagram for EIS test.
Figure 7: TGA curve/decomposition temperature of RA Exp1, 2K Epoxy and Moisture cured Urethane 1 & 2.

Potential Use In Extreme Conditions

Thermogravimetric analysis (TGA) was performed on RA Exp1, 2K Epoxy and moisture-cured urethane 1 & 2. The results indicate that RA Exp1 has comparatively higher decomposition temperature of 463.74 °C, whereas the decomposition temperature of other coatings ranges from 430-440 °C (Figure 7). This study confirms that the RA Exp1 can potentially be used in an environment where coatings are exposed to extreme conditions such as high heat i.e. boilers, chemical processing equipment, pressurized vessels etc.

High-Performance Two-Coat Corrosion Protection

The novel technology represents a dramatic enhancement in the corrosion resistance of metal substrates such as: pre- treated aluminium, zinc-nickel treated cold rolled steel, lightly rusted steel and zinc phosphate treated cold rolled steel coated with RA Exp1. Results demonstrate better face blister resistance, scribe creep resistance and overall better corrosion resistance per ASTM B117 than all other systems tested in this scope of work. The higher decomposition temperature per TGA analysis indicates a potential use of RA Exp1 for high temperature applications. The reaction kinetics of different vinyl polymerisation reactions and oxidative cure of RA Exp1 are not fully defined and still remains a subject of investigation.

The potential applications for this technology include: high-performance protective coatings for maintenance and repair application, automotive refinishing, industrial application, product finishing, offshore application such as oil rigs and refineries, the ACE industry, as well as boilers, chemical processing equipment and pressurised vessels.

In conclusion, this new generation of innovative protective coatings and superhydrophobic protective coatings provide the industry unsurpassed corrosion protection in a two- coat system.

3 questions to Atman Fozdar

What temperature do you recommend for curing to achieve an optimal effect? Coating can be cured at ambient temperature similar to how most coatings are cured for maintenance and repair applications in the field but cure can also be accelerated by thermal bake. For ambient condi- tions, full properties are achieved after 7 days.

Did you test the laboratory results under reality conditions? Subject coating has been applied on multiple substrates such as cold rolled steel, zinc phosphated cold rolled steel, hot rolled steel, 2024 & 7075 Aluminum pretreated with hexavalent chrome sealer, Cadmium treated panels (used in aerospace) along with zinc-nickel treated substrates (used in aerospace and automotive). Acceler- ated properties such as UV-A exposure, ASTM B117 salt spray, Cleveland condensing humidity test along with real life exposure in some of the warmer climate regions near coastal areas are currently being tested.

Are the high temperature loaded films you mentioned still corrosion resistant? Coated objects exposed to temperature in excess of 350–400 °C but less than 450 °C along with saturated steam exposure are performing well after few weeks of salt spray exposure (ongoing test). However, this test was performed in a controlled lab condition. Field evaluation is still a subject of investigation.

[1] MertenB.,CoatingevaluationbyElectro- chemical Impedance Spectroscopy (EIS) Report “ST-2016-7673-1” 2015.
[2] NACEInternational-http://inspectioneering. com/news/2016-03-08/5202/nace-study- estimates-global-cost-of-corrosion-at-25-trillion- ann. 2016
[3] Simpson J. et al. 2015 Rep. Prog. Phys. 78 086501.

Featured photo: Source: Nikolay Zaburdaev – stock.adobe.com

Take Flight with Exterior Aerospace Coatings

Aerospace coatings for exterior applications require a demanding set of performance attributes to provide acceptable performance from both a functional and aesthetic standpoint. In many cases the cost of a new commercial aircraft can be over $300 million with the expectation of lasting several decades with flight times of 4,000 hours or more on an annual basis. According to GMI, the aerospace coating market size is estimated to surpass $1 Billion in sales by 2024.

Read about the challenges of formulating aerospace exterior coatings in the Prospector Knowledge Center.

  • Ability to maintain adhesion and flexibility when subject to rapid temperature changes from 120F to – 70F in a matter of a few minutes
  • Resistance to hydraulic fluids including Skydrol, diesel fuel, lubricating oils and deicing fluids
  • Resist degradation when exposed to intense UV light at high altitudes
  • Repeated dry hot and cold moist cycles
  • Outstanding corrosion resistance as aircraft are often operated in marine and industrial environments
  • High degree of flexibility and resistance to stress as a result of turbulence, vibration and wing flexing
  • Abrasion and erosion resistance and paint from dirt and sand at sub and supersonic speeds
  • Infrared (IR) reflectivity (military applications)
  • Low density (offers weight savings)
  • Icephobic
  • Low COF

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The substrate for the fuselage and aircraft skin is predominantly AA 2024 aluminum. AA 2024 is an alloy of copper and aluminum. The copper provides an increase in the strength to weight relationship, however it is also detrimental to corrosion resistance. Weight reduction is an enormous driving force in new aircraft design as it equates to fuel savings, speed and range. Composites, fiber metal laminates and aluminum-lithium alloys are being used on an increasing basis.

A number of cleaning/pretreatment types (historically hexavalent chrome-based) provide a thin protective layer to improve corrosion resistance as well as receptivity of subsequent coats as it increases surface tension and polarity of the surface.

  • Organic Coatings typically include a primer, pigmented basecoat and a clearcoat.
  • Primers are typically organic solventborne and waterborne two-component epoxy-polyamine/polyamide types containing extenders, additives, catalysts and are further fortified with corrosion inhibitive pigments.

Common types of corrosion on aircraft include filiformpitting, intergranular, exfoliation, stress cracking, galvanic and crevice corrosion. All these types of corrosion are exacerbated by moisture, salt, thermocycling and direct contact of metals differing in metallic content.

Common corrosion inhibitive pigments historically used in aerospace primers include barium chromate and strontium chromate. Epoxy resins for the most part are combinations of bisphenol A and bisphenol F types. When formulated with suitable crosslinking agents (normally amine or amido-amine type) epoxy-based primers provide excellent adhesion, corrosion resistance and chemical resistance.

Filiform Corrosion on Coated Aluminum - learn about exterior aerospace coatings in the Prospector Knowledge Center.
Figure 1. Images of Filiform Corrosion on Coated Aluminum
Cross-section of Aerospace Coating Layers - learn about exterior aerospace coatings in the Prospector Knowledge Center.
Figure 1a. Cross-section of Aerospace Coating Layers
Typical epoxy resins and epoxy functional reactive diluents used in aerospace primers. Learn more about aerospace exterior coatings in the Prospector Knowledge Center.
Figure 2. Typical epoxy resins and epoxy functional reactive diluents used in aerospace primers
Reactions of epoxy resins with amino functionalities - learn about exterior aerospace coatings in the Prospector Knowledge Center.
Figure 3 Reactions of epoxy resins with amino functionalities

Aerospace exterior topcoats are two-component urethane types comprised of hydroxyl functional resins [polyesters, acrylics or fluorinated ethylene vinyl ethers (FEVE)] reacted with isocyanate prepolymer(s). Typical curing reactions are as follow:

Reactions of polyols with isocyanate functional cross-linkers - learn about exterior aerospace coatings in the Prospector Knowledge Center.
Figure 4 Reactions of polyols with isocyanate functional cross-linkers

Due to the demanding requirements of aerospace coating systems, chemists use a stoicheometric excess of isocyanate crosslinker to provide excellent chemical resistance. The excess isocyanate crosslinker reacts with moisture to decarboxylate to form a polyurea upon further reaction. Typically a 50 percent or more stoichiometric excess of isocyanate is used to ensure a high degree of polyurea formation.

Polyureas are known for their superb resistance to aggressive fluids such as Skydrol (an aircraft hydraulic fluid). Polyester polyolsare used primarily in the pigmented basecoat portion of the two component polyurethane coating, whereas acrylic polyols and also FEVE-based polyols are primarily used in the clearcoat portion of the polyurethane topcoat.

Clearcoats are further fortified with both UV absorbers as well as hindered amine light stabilizers to further protect the coating system from degradation due to exposure to intense upper atmosphere UV light.

Isocyanate crosslinkers are typically derived from hexmethylene diisocyante (HMDI) and/or isophorone isocyanate (IPDI). The former type provides flexibility, whereas the latter can provide improved hardness.

Biuret formed from the reaction of three HMDI molecules - learn about exterior aerospace coatings in the Prospector Knowledge Center.
Figure 5 Biuret formed from the reaction of three HMDI molecules
Isocyanurate formed from the reaction of three HMDI molecules - Prospector Knowledge Center
Figure 6 Isocyanurate formed from the reaction of three HMDI molecules
Uretdione formed from two HMDI molecules, as used in exterior aerospace coatings
Figure 7 Uretdione formed from two HMDI molecules
Isophorone Diisocyanate - learn about its use in exterior aerospace coatings in the Prospector Knowledge Center
Figure 8 Isophorone Diisocyanate

Isocyanurate-based isocyanate cross linkers provide excellent weathering characteristics when reacted with a suitable polyol resin system and are thus widely used in aerospace topcoats.

Recent innovations and project emphasis in aerospace coatings include chrome-free pretreatment-primers and chrome-free epoxy primers. Drag-reducing topcoats that provide a 1 percent improvement in fuel efficiency can lower fuel costs by $700 million a year, according to the International Air Transport Association (IATA). On average, airlines incur about $100 a minute per flight in total operating costs, IATA says. Therefore, even saving just one minute of flight time could reduce total industry operating costs by more than $1 billion a year and significantly reduce environmental emissions.

Further Reading:

References:

  • Active Protective Coatings, Springer et.al., 2016
  • Organic Coatings Science and Technology, 3rd Edition, Wicks et.al, 2007

Get a Reaction with Urethane Coatings

Polyurethanes coatings have come a long way since their invention by Otto Bayer and coworkers in 1937. Depending on the choice of oligomeric and polymeric materials, these paints are used in a variety of demanding high performance applications due to their versatility. They can be hard or soft, flexible or rigid, resistant to chemicals and provide excellent adhesion.

Polyurethane properties and applications

  • Outstanding moisture and corrosion resistance
  • Flexible primers and topcoats
  • Weather resistance (aliphatic polyisocyanate with suitable durable polyol)
  • Resistance to acid rain and other chemicals
  • One component
  • Two component
  • Waterborne one component bake finishes
  • 100% solids
  • Powder coatings
  • Waterborne ambient cure two component finishes

Polymeric and isocyanate prepolymer components include one or more isocyanate prepolymers and one or more polymeric or oligomeric components containing hydroxy functionality or other active hydrogen group. Isocyanates are reactive with functionalities which include:

  • Hydroxy
  • Amino
  • Imino
  • Ketimene
  • Carboxyl (forms CO2)
  • Urethanes
  • Ureas
  • Acetoacetylated resins

The active hydrogen for exterior weatherable coatings is normally an aliphatic hydroxyl group in a polyester or acrylic polymer. Alcohols and phenols react with an isocyanate to form urethanes.


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Prospector can help speed along your research with technical datasheets and access to global equipment suppliers.

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Urethane reactions

In the following reaction, R1 and R2 can be aliphatic or aromatic.

R1-R2-aliphatic-or-aromatic formula - Learn more about polyurethane coatings

The urethane reaction is reversible at higher temperatures. For baking systems such as those using blocked isocyanates, excessive bake temperature can result in embrittlement, color change and a decrease in moisture and corrosion resistance.

As a general rule, reaction rates for urethane formation is in the following order:

primary hydroxyl > secondary hydroxyl > tertiary hydroxyl. The reverse reaction rate is the inverse of the forward reaction. For example urethanes from tertiary hydroxyls are relatively unstable.

Once formed, urethanes can react further with isocyanates to form allophanates:

Allophonates formula - Learn more about polyurethane coatings

Other ambient cure reactions of an isocyanate and polyol follow:

isocyanate-polyol - Learn more about polyurethane coatings

As illustrated above, the desired crosslinking reaction between a polyol and an isocyanate to form a polyurethane involves multiple competing reactions. For this reason, two-component formulations with polyol in one component and isocyanate in a second component are normally formulated with a 10% or more stoichiometric excess of isocyanate to overcome competing reactions with moisture and other possible reactants.

Polyurethane catalysts

Catalysts for polyurethanes include tin based carboxylates such as dibutyl tin dilaurate, dibutyl tin octoate or tertiary amines such a DABCO [N2(C2H4)3]. For toxicity concerns, there are also tin-free catalysts based on bismuth neodecanoate, bismuth 2-ethylhexanoate or other metal carboxylates.

Isocyanates and polyisocyanates

There are multiple aliphatic and aromatic polyisocyanates available for use in ambient cure two-component solvent born, 100% solid liquid or powder, as well as waterborne paints. Blocked isocyanates are used in single component baked coatings as they unblock at an elevated temperature to activate the isocyanate group. The reaction sequence is first unblocking and then addition. Polyurethanes formed from aromatic isocyanates are used primarily in primers and interior coatings due to poor light stability, but excellent moisture and corrosion resistance.

Common aliphatic and aromatic polyisocyanate building blocks include:

polyisocyanate building blocks formulas - - Learn more about polyurethane coatings

HDI and IPDI are used to synthesize higher molecular weight isocyanate prepolymers which may include isocyanurates, allophanates and uretdiones to improve hygiene, handling and weathering properties.

Isocyanates can be blocked to form a stable material for use as a crosslinker in single component polyurethane coatings. Blocked isocyanates are used extensively in powder, waterborne and high solids baking finishes for coil primers, automotive coatings and electrodeposition coatings. Common blocking agents include 2-ethylhexanol, e-caprolactone, methyl ethyl ketoxime and 2-butoxy ethanol. When mixed with a polyol, blocked isocyanates are stable until they reach the unblocking temperature and then eliminate the blocking agent and react with the polyol to form a polyurethane.

Waterborne two component urethane coatings can be made using water dispersible isocyanates. Water dispersible IPDI or HDI based isocyanates are commercially available and are made by reacting a portion of the isocyanate groups with polyethylene glycol monoether. The polyisocyanate is then added into a separate dispersion containing the polyol to form separate dispersed particles that crosslink and form a film.

Iso-free technology

Isocyanate-based technology has come under increased scrutiny as exposure to isocyanates can cause asthma and other respiratory issues. Occupational asthma has overtaken asbestosis as the leading cause of new work-related lung disease. Isophorone free technology provides polyurethane formation without exposure to free isocyanate. In the last few years isofreetechnologies have emerged that do not utilize isocyanate crosslinkers to form  a polyurethane and thus eliminate isocyanate exposure. Isofree 2K technology utilizing polycarbonate and polyaldehyde for example includes improved sprayable pot life and rapid cure and early hardness. Technologies that form polyurethanes without the use of an isocyanate crosslinker follow:

  1. Hexamethoxy methyl melamine + Polycarbonate → Polyurethane
polyurethane formula - Learn more about polyurethane coatings
  1. Polycarbonate + Polyamine → Polyurethane
polyurethane formula - Learn more about polyurethane coatings
  1. Polycarbamate + Polyaldehyde → Polyurethane
polyurethane formula - Learn more about polyurethane coatings

The formation of polyurethanes in reactions #1 and #2 are sluggish at room temperature, whereas the reaction rate of #3 that utilizes the crosslinking reaction of a polycarbonate and a polyaldehyde is more facile. Polyurethane formation by this reaction route provides a longer sprayable pot life and at the same time a faster reaction rate after application than that provided by the use of an isocyanate crosslinker.

Sources:

Prospector Knowledge Center and Search Engine

Polyurethanes. (2017). The Essential Chemistry Industry – online.

Mahendra, Vidhura. (2016). Foam making via pine resins. 10.13140/RG.2.1.2065.0004.

Wikepedia. Polyurethane.

John Argyropoulos, Nahrain Kamber, David Pierce, Paul Popa, Yanxiang Li and Paul Foley. Dow Isocyanate Free Polyurethane Coatings – Fundamental Chemistry and Performance Attributes, European Coatings Conference, April 21, 2015.

Zeno W. Wicks Jr., Frank N. Jones, Socrates Peter Pappas, Douglas A. Wicks. (2007). Organic Coatings: Science and Technology, Third Edition.

Wiley, Jones e.al. (2017) Organic Coatings, Science and Technology, Third Edition.

Cause and Defect: Evaluating and Testing the Weathering of Coatings

Exterior weathering can have a dramatic effect on the aesthetic, functional and physical properties of coatings that can include chalking, film erosion, cracking, color change, etching, blisters, peeling, spotting, and loss of hardness, flexibility (increase in glass transition temperature, or Tg), gloss, and adhesion. Multiple formulation issues influence the performance of coatings in a given exterior environment and include:

  • Resin Type
  • Crosslinker Type
  • Pigment/color
  • Pigment type
  • Pigment to binder ratio
  • Presence of Catalyst
  • Additive selection

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What impacts exterior coating weathering?

Some of these factors will be covered in more detail than others to the degree they influence weathering. The major issues impacting exterior weathering include: photooxidation (presence of oxygen and light) and hydrolysis due to the effects of moisture, heat and light. The former can be mitigated to a degree with the proper use of UV absorbers to reduce exposure of the polymer matrix to UV light, antioxidants and hindered amine light stabilizers (HALS) to reduce the effects of associated oxidative degradation.

Both photooxidation and hydrolysis are exacerbated by an increase in temperature as both are thermally activated. Environments high in airborne moist salt and/or acid rain (high sulfate, nitrate) and ozone accelerate the hydrolysis and degradation of resin systems and accelerate color change due to acid attack of pigments.

By far the major process that influences film degradation of polymeric coatings is photooxidation. Oxidative degradation proceeds by hydrogen abstraction from the polymer through an autocatalytic process. Accordingly, to achieve excellent weathering, avoid or at least minimize functional groups in the polymer that are more vulnerable to hydrogen abstraction. Following is a general order of functional group resistance to oxidative degradation of activated methylene groups (- CH2) between double bonds or adjacent to amine groups being the worst:

Order of functional group resistance to oxidative degradation of activated methylene groups - learn more about testing coating weathering in the Prospector Knowledge Center.

Accordingly, in general fluoropolymers and siloxanes are more durable than polyesters or urethanes followed by resin systems high in aromatic content, and amine groups being the least durable. The later types include aromatic epoxies.

Characteristics of UV Stabilizers include absorption and quenching. UV absorbers act by absorbing radiation in the wavelength region where the polymer system absorbs thus acting to shield the resin from degradation. Ideally UV Stabilizers should have a high absorption in the UV region from 295 to 380nm to provide protection for the polymer from degradation. The most effective UV stabilizers are also more permanent thus ensuring longer life once incorporated into a paint system.

UV stabilizers convert the absorbed UV energy into heat, such as that with 2 – hydroxy benzophenone:

Chemical formula for 2 – hydroxy benzophenone - learn more about testing coatings for weathering in the Prospector Knowledge Center.
Chemical formula for 2 – hydroxy benzophenone

Antioxidants are classified into two groups of preventative (peroxide decomposers and chain breaking antioxidants). Peroxide decomposers include sulfides and phosphites. Chain breaking antioxidants disrupt the chain propagation step of autoxidation. Organic materials react with molecular oxygen in a process called “autoxidation“. Autoxidation is initiated by heat, light (primarily in the UV region), mechanical stress, catalyst residues, or reaction with impurities to form alkyl radicals. The free radical can, in turn, react and result in the degradation of the polymer such as depicted below:

Image of Autoxidation Cycle: Degradation - learn more about how to test coating weathering in the Knowledge Center.

Hindered Amine Light Stabilizers (HALS) function both as chain breaking antioxidants as well as  complexing agents for transition metals. For coatings that provide excellent durability, the rate of Hydrolysis is normally much lower than that of photooxidation.

The rate of hydrolysis for functional groups is esters>carbonates>ureas>urethanes>ethers.

For crosslinked products, melamines hydrolyze at a faster rate than that of aliphatic urethanes.

As most systems used in exterior applications contain pigment (including basecoat/clearcoat systems used in exterior automotive topcoats); pigment selection, color as well as pigment volume concentration (PVC) all contribute to the durability of the paint system. PVC selection is somewhat dictated by gloss level, color requirement and film thickness necessary for acceptable hide (color uniformity over the substrate).

In paint systems dependent on protection provided by pigment for light stability, durability is more dependent on relatively small variations in PVC. The relationship between color and weathering can be very complicated. For example, darker colors tend to absorb more radiant energy and thus the heat absorption coefficient for darker colors not using solar reflective pigments is higher, contributing to higher temperatures of the coating exposed to exterior radiant energy:

Chart of heat accumulation of organic coated substrates using conventional pigments in sunlight - learn more about testing coating weathering in the Prospector Knowledge Center.

Higher temperatures contribute to higher degradation rates, however darker colors (brown/black) absorb more UV/Visible light energy and thus help protect the polymer system from degradation. Accordingly the use of a resin system prone to oxidative degradation at higher temperatures will provide poor weathering especially in dark colors.

Pigment selection within a class of colors can have a tremendous effect on the durability within a class of polymers. Pigments used for color and hiding can be divided into two general classes including inorganic and organic.

Inorganic pigments as a class are more resistant to degradation and chemicals than are organics. Some of the durable inorganic pigments include acid resistant aluminum flake, micaceous iron oxide, yellow, brown and red iron oxides.

The most durable inorganic pigments are Ceramic pigments. Ceramic pigments are mixed metal oxides. As these pigments are fully oxidized they are very resistant to chemicals and oxidation. As many bright colors require organic pigments, such pigments are a necessity. Many organic pigments can provide exceptional resistance to exterior degradation and are used extensively in automotive basecoats.

How to evaluate coating weathering

The best way to evaluate weathering is by natural exposure in the color, environment, gloss level and exposure angle the coating will be used in. As that is not practical for the introduction of new coatings, accelerated weathering is a necessity.

South Florida weathering is normally the most accepted means to determine accelerated natural weathering of a coating. For example: 5 degrees horizontal south facing for automotive applications or 45 or 90 degrees facing south or north respectively for architectural applications.

Marine environments are also commonly used for paint systems to evaluate corrosion protection or resistance to biological growth. Although South Florida weathering provides a good indication of the projected durability, there is always a desire to further reduce the time required to predict the durability of a coating to an environment high in UV, moisture, and high temperature.

A few of the other commonly used methods to determine accelerated weathering include ASTM D 4587 (QUV weathering) and ASTM G155/ASTM D7869 (Xenon Arc). These accelerated weathering devices provide a combination of cycles of intense UV light, high temperature and high humidity. There are a number of articles detailing the correlation or lack thereof with natural weathering including new instruments and processes that profess to provide a better correlation to natural weathering.

Graph of Xenon with daylight filter - learn more about how to evaluate coating weathering in the Prospector Knowledge Center.
Chart of coating weathering results - learn more about how to evaluate coating weathering in the Prospector Knowledge Center.

Sources and further reading:

The Source for Chemical Coatings Consulting