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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

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

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

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

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

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).

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.

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 Adhesion) utilizes 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.).

Sources:

• Organic Coatings, Science and Technology, Frank N. Jones et.al., Wiley & Sons, 2017
• Prospector Knowledge Center
• www.surface-tension.de
• Science Direct
• Science & Technology AJ Kinloch, Chapman & Hall
• CSCScientific.com
• ASTM Standards
• www.defelsko.com

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-¹ 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/cm² / sec^-1 or dyne-sec / cm² or 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

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

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

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

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

Fig. 4 Rheology profile for multiple paint processes

Fig. 5 Viscosity requirements for mill base formulation

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

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 modification. In 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

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.

• 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 filiform, pitting, 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.

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:

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.

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:

• As the Wheel Turns: Aluminum Cleaning, Passivation, Corrosion Protection and Monitoring
• Fundamentals of Corrosion Protection
• Understanding Corrosion Inhibitive Pigments
• Chemistry of Resins and Hardeners

References:

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

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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Urethane reactions

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

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:

Other ambient cure reactions of an isocyanate and polyol follow:

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:

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

1. Polycarbonate + Polyamine → Polyurethane

1. Polycarbamate + Polyaldehyde → Polyurethane

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.

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 T_g), 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:

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:

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:

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:

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.

Sources and further reading:

• Prospector Search Engine
  • UV Absorbers
  • antioxidants
  • ceramic pigments
  • mixed metal oxide
  • HALS
  • iron oxide
  • UV stabilizer
• Organic Coatings, Science and Technology, Frank N. Jones et.al., Wiley & Sons, 2017
• Exterior Durability of Organic Coatings, Eric V. Schmid, FMJ International, 1988
• The Right Choice, Jim Regan, Q Lab
• Prospector Knowledge Center: Resin Fundamentals and Their Effect of Coatings Performance, R. Lewarchik
• Prospector Knowledge Center: Effect of PVC, R. Lewarchik
• Prospector: Architectural Coatings that Reduce Heating and Cooling Costs, R. Lewarchik
• SpecialChem: Antioxidants to Prevent Polymer Oxidation

Silicate coatings are alkali metal silicates that are made from naturally occurring materials such as sand and alkali. Alkali metal silicates are derived from a combination of silica (SiO₂) and a carbonate of lithium, sodium or potassium to produce a silicate (SiO₂/Na₂O). Depending on their formulation, these remarkable coatings can have multiple benefits including:

• Not petroleum based
• Outstanding durability
• UV resistant
• Acid rain resistant
• High hardness
• Exceptional wear resistance
• Outstanding hardness
• Non-flammable
• Adhere to multiple substrates
• High moisture and gas permeability (can be a benefit or a disadvantage)
• Chemically bond to mineral surfaces
• Heat Resistance (most silicate based paints have a softening point of ~ 1,200F)
• Heat Resistant paint for metals (silicates mixed with copper, nickel, chromium or stainless steel powders)
• Good chemical and physical properties
• Zero VOC

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Types of silicate-based coatings

Types of silicate-based coatings include silicate, silicate-organic emulsion and lastly sol-silicate.

Soluble silicates include those of the Group 1A elements of the Periodic Table (Li, Na and K). As Silicates are based on alkali metal oxides and silica, their solutions are alkaline. As the molar weight ratio of the silicon:alkali metal increases, the pH decreases:

Accordingly, when blending alkali metal silicates with organic emulsions, it is important to use higher ratios of silicon to alkali metal to achieve the best stability and a workable pH of 8 – 10 for most organic-based emulsions.

Viscosity of sodium silicate solutions is a function of concentration, density and ratio of sodium: silicon. Higher or lower ratios increase viscosity with a minimum viscosity reached at a 2.0 weight ratio.

From a structural standpoint, waterborne silicates are glasses that have a wide variety of molecular structures in which the anions are monomers, dimers, trimers, branched chains, and ring structures, as well as other three dimensional networks. Cations of alkali metals (Li⁺, Na⁺ and K⁺) attach to the anions (Si – O – ) to create a complex alkali silicate.

There are two equilibria in an alkali silicate solution, that includes an acid-base equilibrium:

As well as a condensation polymerization-depolymerization equilibrium:

Irreversible reactions also take place with polyvalent cations such as Ca⁺⁺ or may also include Mg⁺⁺, Fe, or Mn.

The ratio of alkali metal oxide to silica has a significant effect on coating properties as illustrated in the table below:

The higher ratio (High SiO2 low NaCO3, e.g. 3.75 to 1) gives:	The lower ratio (Low SiO2 High NaCO3, e.g. 2 to 1) gives:
Lower viscosity	Higher specific weight
Faster drying speed	Greater solubility
Faster curing speed	Higher pH value
Increased susceptibility to low temperatures	Greater susceptibility to water influence
Higher chemical resistance of coatings	Higher tack and binding power

Commercially available silicates are normally produced in ratios of 1.5 or higher. Coatings based on sodium silicate can be used and require a catalyst for ambient cure, but are susceptible to efflorescence. Solutions of sodium silicate can react or cure with dissolved polyvalent ions including Ca++, Al⁺⁺⁺ and Mg++ to form insoluble silicates.

• Potassium silicates are self-curing, however the reaction is slow.
• Lithium silicates have low water solubility and are used to minimize water soluble by-products and efflorescence.
  • Efflorescence is a whitish, powdery deposit on the surface of a material (stone, concrete, brick and mortar) caused from mineral-rich water percolating to the surface through capillary action. Efflorescence usually consists of gypsum, salt, or calcite.

Mineral calcium carbonates (e.g. calcite) exhibit low reactivity with soluble silicate, whereas precipitated calcium carbonate provides high reactivity. The viscosity of sodium silicates is very high, whereas colloidal silicas (stabilized silica particles less than < 100nm in size) have viscosities closer to that of water. pH has a major impact on the viscosity of colloidal silicas and form gels at a pH < 7 and a Sol when a pH is >7. Liquid sodium and potassium silicates also can be reacted with a variety of acidic or heavy metal compounds to produce solid, insoluble bonds or films.

Neutralizing an alkali silicate with acidic materials (e.g., aluminum sulfate) polymerizes the silica and forms a gel. This produces a bond or film on surfaces where gellation occurs. Chemical setting agents that can be used in this manner include: mineral and organic acids, carbon dioxide (CO2) gas, and acid salts such as sodium bicarbonate and monosodium phosphate (NaH2PO4).

Silicate-emulsion paints comprise a low level of a polymeric organic emulsion (~5%) with an alkali silicate. The emulsion helps to enhance water resistance until the silification reaction is complete, which can take weeks. Higher levels of organic emulsions are generally incompatible.

Typical components of a silicate-emulsion paint can include:

• organic additives like compatible surfactants
• small amounts of suitable coalescing solvents
• thickeners (e.g. Hydroxyethylcellulose, or HEC), stabilizers and modifiers
• emulsions that are stable at higher pH that may include:
  • aqueous dispersions of polymers such as:
    • styrene-butadiene
    • polystyrene
    • neoprene
    • polyvinyl chloride
    • polyvinyl acetate
    • acrylonitrile copolymers
    • acrylic polymers and copolymers
  • inorganic binders such as potassium silicate and filler pigment
  • inorganic alkali resistant pigments

As silicate paints are not generally flexible, they can be flexibilized by the addition of 1 to 5% by weight of glycerine or other polyhydric alcohols. Up to 30% of sorbitol can be used, provided the silicate solution is diluted to avoid excessive thickening.

Rubber lattices can also be employed as plasticizers. Incorporation of finely ground clays and similar fillers will improve flexibility to some extent. Silicate emulsions paints can also be formulated for use on aluminum, galvanized steel, steel, stone, brick, concrete, and previously painted surfaces that used an emulsion paint.

Sol-silicate paint is a combination of silica-sol and potassium silicate. An organic binder is incorporated at a percentage of 10% or lower. As opposed to most other silicate paints, sol-silicate paints bond to non-mineral substrates through both physical and chemical bonds. Silica sols are dilute solutions of dissolved silica that are at an acidic pH.

Sources and further reading:

• Prospector Knowledge Center and Search Engine
  • silicates
  • sodium silicate
  • lithium silicate
  • potassium silicate
  • silicon emulsion
  • silicate emulsion
  • Acronal S-559
  • colloidal silica
• Bulletin 12-31, PQ Corporation, Bonding and Coating Applications of PQ soluble Silicates
• Book One. Waterborne Silicate Coatings, Rev 3, July 2016, Ken Marx

Ambient curing by definition relies on conditions that are available in an ambient environment such as moderate temperature, natural light, moisture and air. From the use of ochre based cave paints 40,000 years ago to that used by the early Egyptians about 4,000 years ago that comprised pigment, wax and eggs; humans have been searching for and developing new chemistries and ingredients to provide improved performance of coatings applied at ambient conditions.

Comprised of natural occurring pigments and drying oilsfrom linseed, poppy seed, walnuts and safflowers, the first ambient cure crosslinked paints were first used by Indian and Chinese painters between the fifth and 10th centuries.

The proper use of curing agents (either or both single or two component types) can provide improved:

• Chemical resistance
• Moisture resistance
• Adhesion
• Hardness
• Corrosion resistance
• Weather resistance

This article will cover only the general considerations of ambient curing agents with emphasis on newer chemistries or chemistries less often utilized. As there are previous Prospector articles concerning conventional epoxy two component (2K) coatings, two component coatings polyol-isocyanate technology and finally moisture cure silane functional crosslinkers and coupling agents, these technologies will not be discussed herein.

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Isocyanate-free polyurethane chemistry

According to the California Department of Public Health, exposure to isocyanates can cause asthma. Occupational asthma has overtaken asbestosis as the leading cause of new work-related lung disease. In the last few years isofree technologies 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, rapid cure and early hardness. Technologies that form polyurethanes without the use of an isocyanate crosslinker follow:

1. Hexamethoxy methyl melamine + Polycarbonate ⇒ Polyurethane

2. Polycarbonate + Polyamine ⇒ Polyurethane

3. Polycarbamate + Polyaldehyde ⇒ Polyurethane

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.

Ketimine-Epoxy

One approach to provide stable epoxy-amine single component coatings is to utilize a blocked amine crosslinker. Primary amines react with ketones to form ketimines. Ketimines do not readily react with epoxy groups. In the presence of water, ketimines release the free amine plus ketone which is the reverse reaction of ketimine formation. Normally methyl ethyl ketone is used which upon application volatilizes quickly under ambient conditions, the amine then reacts with the epoxy to form a cured film. A moisture scavenger additive can eliminate the reaction with water prior to application.

Ketimine-epoxy systems are indefinitely stable in the absence of water and can thus permit one component systems.

Crosslinking with unsaturated groups

• Acrylated oligomers can be used as a crosslinker to crosslink polyfunctional amines through a Michael Addition Reaction. As this reaction is fast, blocked amines can be used (ketimines). Once the ketimine is unblocked in the presence of moisture, it forms a primary amine that adds to the acrylate for the reaction of a primary amine and an acrylate. See illustration below.

• Acrylated oligomers can also be crosslinked using the Michael Addition Reaction with acetoacetoacetylated resins and their enamine analogues.
• Vinyl polymerization – Coatings utilizing acrylated and/or methacrylate oligomers and suitable unsaturated polyesters (using fumerate and/or maleate groups) can be utilized in two component systems with the addition of a suitable free radical initiator such as methyl ethyl ketone peroxide and accelerators such as cobalt napthenate and dimethylaniline.

Other common crosslinking reactions utilized in ambient cure coatings

HardenerCross-linker Functional Group	ResinCross-linkable Group	Cross-linked group
Polyaziridine	R-COOH(carboxyl)	Acetyl urea
Silane Triethoxy silane and aliphatic epoxy	Dual self-cure mechanism	Siloxane & epoxy ester
Carbodiimide R-N=C=N-R	R-COOH	N-Acyl Urea
Isocyanate prepolymerR-NCO	R-OH(hydroxyl) R-NH2(amino generated from reaction of water with isocyanate)	Urethane  Urea
Hydrazide	R-C=OKetone	Hydrazone

Sources:

• Prospector Knowledge Center: Chemistry of resins and hardeners
• Prospector Knowledge Center: Superior performance with organosilane chemistry
• Prospector Knowledge Center: Patently innovative latest polyurethane technologies
• Prospector Knowledge Center: Polyisocyanates deep dive
• Prospector Knowledge Center: Reactive silanes for enhancement of coating performance
• Prospector Knowledge Center: Ambient cured latex paints and how to improve performance
• California Department of Public Health: Isocyanates: Working Safely [PDF]
• Wikipedia: Michael reaction
• Web exhibits.org
• Dow Isocyanate Free Polyurethane Coatings, European Coatings Conference, April 21, 2015
• Organic Coatings, Science and Technology, Third Edition, Wiley, Wicks e.al. 2007

Coated surfaces can impart  a wide range of affinity related to water, from hydrophilic (water loving), to hydrophobic (water repelling) to superhydrophobic (super water repellency). These surface characteristics are obtained by the proper combination of surface morphology at the micro and/or the nanoscale level, combined with a low surface energy material.

Superhydrophobicity and the lotus leaf

A prime example of superhydrophobicity in nature is the lotus leaf. The lotus leaf has a microstructure  comprising small protuberances or spiked papillae 10 – 20 microns in height and 10 – 15 microns in width which have a second hydrophobic wax layer. The combination of a structured surface combined with a low energy wax provides superhydrophobicity to the surface. To fully explain and quantify hydrophobicity, it is necessary to define the relationship between contact angle and the hydrophobic/hydrophilic character of a surface.

Contact angles of 150° or more and are called superhydrophobic – meaning that only two to three perfect of the surface of a water droplet is in contact with the surface. Since the surface contact area is less than 0.6 percent, this provides a self-cleaning effect. The ramifications of imparting lotus leaf water repellency characteristics to a coating surface has profound performance implications which can include the following:

• Self-Cleaning – Contaminants that fall on a superhydrophobic/hydrophobic surface are removed as water droplets will roll off.
• Improved moisture resistance – Improved blister resistance and gloss retention
• Improved corrosion resistance – Lowering moisture penetration reduces or even eliminates water and soluble salt penetration to the metal substrate which greatly slows the onset of corrosion.
• Extended life cycle for coating and substrate – Increased coating weatherability and resistance to the penetration of soluble salts and moisture positively impacts the longevity of the coated article.

The role of surface tension

We have discussed the role that surface morphology plays in imparting hydrophobicity; the other  critical component for hydrophobicity is surface energy.

• Surface tension is the elastic tendency of liquids that make them acquire the least surface area possible.
• Surface tension is measured along a line, whereas surface energy is measured along an area.

Components of surface tension mainly include dispersive and polar, hydrogen bonding and acid-base contributions. In general lower surface energy materials provide higher hydrophobicity. Table 1 and 3 lists the Surface Free Energy of several polymer types and modifiers, respectively, used in coatings, whereas Table 2 provides surface tensions of commonly used solvents in coatings.

Polymer	Surface Free Energy mN/m
Polyhexafluoropropylene	12.4
PTFE	19.1
PDMS	19.8
Parafin Wax	26.0
Polychlorotrifluoroethylene	30.9
Polyethylene	32.4
Polyvinyl Acetate	36.5
Polymethylmethacrylate	40.2
Polystyrene	40.6
Polyvinyldene Chloride	41.5
Polyester	43 – 45
Polyethyleneterephthalate	45.5
Epoxypolyamide	46.2

Table 1 – Surface Free Energy of Polymers

Solvent	Surface TensionDynes/cm
Water	72.8
Toluene	28.4
Isopropanol	23.0
n-Butanol	24.8
Acetone	25.2
Methyl propyl ketone	26.6
Methyl amyl ketone	26.1
PM acetate	28.5

Table 2 – Surface Tension of Solvents

Material Identity	Critical Surface TensionmN/m
Heneicosafluoro-dodecyltrichlorosilane	6-7
Heptadecafluorohexyl--trimethoxy Silane	12.0
PDMS	19.8
Octadecyltrichlorosilane	20-24
Nonafluorohexyl-trimethoxysilane	23

Table 3 – Surface Free Energy of Potential Surface Modifying Agents

When two different liquid materials are applied to a solid surface, the liquid with the lower surface tension will flow or wet out on the solid surface, for example polyethylene, more so than the liquid with the higher surface tension. For example, water (surface tension 72.8 Dynes/cm) will form a higher contact angle than will Toluene (surface tension 28.4 Dynes/cm).

Thus far, we’ve defined the factors that contribute to the hydrophobicity, or the lack thereof, including contact angle, surface structure, and why most organic solvents tend to wet a surface better than water as a consequence of their lower surface tension. The next segment will concentrate on how to impart greater hydrophobicity to a coating system, especially from a surface perspective.

Maximizing surface hydrophobicity

To maximize the surface hydrophobicity of a coating, the surface energy should be as low as possible. A low surface energy, coupled with an appropriately structured surface, maximizes hydrophobicity.

Surface energy has the same units as surface tension (force per unit length or dynes/cm). A high surface tension liquid such as water will have maximum hydrophobicity and thus have poor wetting (high contact angle) over a coating surface that has a lowsurface energy.  As Table II illustrates, surface energy can vary greatly depending on the nature of the surface that comes in contact with water.

For instance, a coating surface that is rich in polydimethylsiloxane (Surface Energy 19.8 mN/m) at the surface will provide a more hydrophobic surface than that of polystyrene (40.6 mN/m). In general terms, to provide the greatest hydrophobicity, the material’s most hydrophobic moiety should be positioned on the surface.

As another example, if an organofunctional trimethoxysilane is used for surface modification, the methoxysilane groups should be engineered to be positioned at the surface. Perfluoro and aliphatic groups at the coating surface offer greater hydrophobicity than that of ester or alcohol groups. Ester and alcohol groups are more polar in nature and thus more receptive to water deposited on the surface. For example, from lowest to highest surface tension:

Providing increased hydrophobicity throughout a properly engineered coating can provide additional attributes such as self-cleaning, improved corrosion and moisture resistance and an extended life cycle for the coating and substrate.

Recent advances in silane technology have enabled the availability of silanes for use in waterborne systems for improved hydrophobicity. Accordingly, resin selection, flattener, extender pigments and opacifier pigments can also be selected to maximize hydrophobicity.

Secondly, formulations utilizing nanoparticles must be tailored to provide proper acceptance rather than as a drop-in to achieve a desired property.

Search UL Prospector^® for hydrophobic raw materials.

You might also be interested in…

• Surface Tension & Surface Energy
• Breaking the tension with surfactants [INFOGRAPHIC]
• Hydrophobic Coatings Explained
• Dispersing & Wetting Hydrophobic Pigments & Fillers in Water-Based Paints to Avoid Pigment Flooding & Floating

Sources

• Prospector Knowledge Center: Hydrophobic Coatings Explained
• Gelest 2016 product literature

Silanes were first discovered and identified in 1857 by German chemists Heinrich Buff and Friedrich Woehler among the products formed by the action of hydrochloric acid on aluminum silicide.¹ Since that time silane chemistry has proven to be a versatile means to enhance performance of organic-based coatings, or to provide siloxane-modified coating systems with a variety of performance characteristics not readily achievable with other technologies.

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Depending on the proper selection of reactive silane, a variety of improved performance attributes can result, including:

• Weathering
• Adhesion
• Hardness
• Flexibility
• Moisture resistance
• Lubricity
• Cross-link density
• Corrosion resistance

Silane and Siloxane Structures:

In the presence of water, a trialkoxysilane can hydrolyze as a first step in the reaction to liberate methanol (for a trimethoxysilane) or ethanol (for a triethoxysilane) and self-condense to form a siloxane or react with available alcohol groups on a pigment, polymer or substrate to provide a siloxane linkage.

Silanes are used in a number of applications to:

• Improve adhesion to inorganic or organic surfaces – Silanes, when added to paints, can enhance adhesion to inorganic surfaces including metals and glass
• Coupling Agents – Silanes are used for coupling organic polymers to inorganic materials including pigments and fillers
• Crosslinking Agent – Selective organofunctional alkoxysilanes can react with organic polymers to provide a trialkoxysilyl group into the polymer backbone. In turn, the silane can then react with moisture to crosslink and form a three-dimensional siloxane cross-linked structure.
• Dispersing Agent – Used to increase the hydrophobicity of inorganic pigments and improve flow characteristics and the ability to be dispersed in organic polymers and solvents.
• Improved hydrophobicity – Selective reactive silanes can be modified to provide superb hydrophobicity (to be discussed more in the sequel to this article)
• Moisture Scavenger – In moisture sensitive formulations, the three alkoxysilane groups can scavenge water by reacting with moisture to form alcohol molecules.
• Pretreatment for metal surfaces – Specialized waterborne silanes for pretreatment of various metal surfaces (e.g. Evonik’s Dynasylan SIVO product group)

A silane that contains at least one carbon silicon bond (CH₃ – Si -) is called an organosilane. Reactive silane is the term used to define compounds that have a trialkoxysilyl group and an alkyl group (R) containing a reactive constituent.

Trialkoxysilyl groups can react directly, or indirectly in the presence of water with hydroxyl groups. As illustrated in Table 1, the other organofunctional group (R) can participate via a crosslinking reaction with another reactive site in a coating.

In regard to the reactions and interactions with a surface, there are many complexities and dependent variables. For example, the rate of hydrolysis of the trialkoxysilyl groups with moisture to form silanol groups (R – Si- OH), which in turn self-condense or crosslink compete with the reaction of the silanol groups with the substrate hydroxyl groups. These competing reactions can vary depending on moisture level, pH, and rates of reverse reactions. as hydrolysis is reversible. Hydrolysis of trialkoxysilyl groups to silanols and the subsequent self-condensation to form a siloxy crosslink (- Si – O – Si -) can be accelerated by the use of a suitable tin catalyst such as dibutyltin dilaurate.

On the other hand, the best catalyst for promoting co-condensation between a resin and -the silicone intermediate are titanate-based catalysts such as tetraisopropyl titanate.

Except for those applications requiring polymerization of a reactive silane into a resin backbone, most of the reactions illustrated in Table I can occur under ambient conditions.

R = Reactive Group onR-Si (-OCH3) or R-Si (-OCH2CH3)	R group Reacts with	Reactive SilaneExample	Trialkoxy Silane Reaction	Application
Amino	Epoxy functionality	3-aminopropyl-triethoxysilane	With –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
Epoxy	Amino functionality	3-glycidyloxypropyl trimethoxysilane	With –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–acrylate	Acrylic resin polymerization	3-methacryloxypropyltrimethoxysilane	Self-crosslink with another silane to form– Si- O – Si – and with –OH on the surface	Moisture cure resins with improved adhesion, physical and environmental performance
N/A	N/A	N-octyltriethoxysilane	Forms– Si – O – Si –	Water repellency, improved hydrophobicity
Vinyl	Vinyl or acrylic resin polymerization	Vinyl-trimethoxysilane	Forms– Si – O – Si –	Moisture cure resins with improved adhesion and film integrity. Also used as a moisture scavenger
Isocyanate	Hydroxyl, Amino or Mercapto	3-isocyanatopropyl-triethoxysilane	With –OH on surface as well as self-crosslink to form – Si – O – Si	Coatings for metallic and inorganic oxides, also moisture cures
SilaneSIVO Sol-Gel				VOC Free Waterborne Surface Treatment for various metals and surfaces

Table I: Reactions of Trialkyloxy Organofunctionalsilanes and Their Applications

Reactive silanes provide utility to improve coating performance in a number of applications, including:

• Pigment wetting
• Improving hydrophobicity and increasing contact angle
• Enhancing adhesion over a number of metallic and inorganic surfaces
• Coupling agent between differential materials
• Scavenging moisture to provide improved stability
• Crosslinking to improve physical and environmental properties

A variety of siloxane-based reactive trimethoxy silane prepolymers are also available with functional groups including acylate, isocyanate, amino, hydroxyl, epoxy and vinyl. These enable a variety of opportunities to improve cross-link density, adhesion, weather resistance, moisture resistance, hydrophobicity and chemical resistance.

Resources

1. Wikipedia: Silane
2. UL Prospector
3. Evonik, ACS Product presentation
4. Organic Coatings, Science and Technology, 3^rd Edition