Waterborne Silicate Coatings: The Ultimate Eco-friendly Coating

Posted on by chemical dynamics

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.

Chemical structures of types of silicate-based coatings. Learn more in the Prospector Knowledge Center.

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:

Diagram of silicon:alkali molar weight ratio. Learn more about waterborne silicate coatings in the Prospector Knowledge Center.

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.

Diagrams of silicate chemical structures - learn about waterborne silicate coatings in the Prospector Knowledge Center.

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

Chemical formula for silicate acid-based equilibrium. Learn about waterborne silicate coatings in the Prospector Knowledge Center.

As well as a condensation polymerization-depolymerization equilibrium:

Chemical formula for silicate condensation polymerization depolymerization equilibrium - learn about waterborne silicate coatings in the Prospector Knowledge Center.

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 viscosityHigher specific weight
Faster drying speedGreater solubility
Faster curing speedHigher pH value
Increased susceptibility to low temperaturesGreater susceptibility to water influence
Higher chemical resistance of coatingsHigher 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.

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SIGNIFICANTLY INCREASED TRANSFER EFFICIENCY

Significantly Increased Transfer Efficiency

Posted on by chemical dynamics

ORIGINALLY PUBLISHED IN EUROPEAN COATINGS JOURNAL 11 – 2018

Enhanced technology for electrostatic spray on nonconductive substrates and complex geometries. By Atman Fozdar, Ronald Lewarchik, Chemical Dynamics LLC, USA, and Vijay Mannari, Eastern Michigan University, USA.

Light vehicles represent an important market for plastics and poly- mer composites, one that has grown significantly in the last five decades. Among other reasons, this is due to being inexpensive com- pared to their metal counterparts, to being able to mould complex geometry, to reduced weight, and to increased fuel economy due to the reduced weight. Various plastics and composites are used for automotive interiors, exteriors, electrical systems, powertrains and engine components. Their complex geometries and the non-conductive nature of the substrate mean that coatings cannot be applied using electrostatic spraying, which limits the methods for applying coatings by the conventional spraying method. A conventional liquid coating spray method causes significant loss in transfer efficiency (about 40- 60% depending on geometry of the substrate), so there is an obvious market opportunity here.

Powder coatings have been identified as the most suitable and eco-friendly coatings for plastics because, for instance:

  • No hazardous Volatile Organic Compounds (VOCs)
  • Higher first-pass transfer efficiency (up to 90-95 %),
  • Overspray can be reused/reclaimed.
  • Superior film properties (tough, durable, hard, scratch resistant)
  • Lower process time and energy requirements.
  • One-step finishing process.
  • However, powder coating plastics and composites gives rise to certain challenges, such as:
  • Applying a powder coating using electrostatic spray on non-con-

ductive plastics and composites.
ą Adhering a powder coating on plastics with low surface energy.
ą Selecting the right powder chemistry that cures at low temperature

due to low heat-deflection temperature of plastics and composites.

We have developed a method that overcomes these challenges.
Use of CAPs eliminates the need for preheating, plasma treatment and chemical etching of plastic substrates while improving both film appearance and application efficiency.

Results At A Glance

  • A new conductive adhesion promoter (CAP) technology for application in a continuous/conveyorised production line, dries quickly.
  • UV curable as well as low temperature cure (LTC) powder and liquid coatings can now be applied uniformly even in re- cess/Faraday cage areas on plastic composites.
  • CAPs work more efficiently at lower film thickness on non- porous substrates.
  • CAPs can significantly increase the transfer efficiency of applying a liquid or powder coating to plastic composites with complex geometry.
Figure 1: Classification of conductive materials by surface resistivity.

Surface Resistivity is Key

Figure 1 classifies conductive materials by surface resistivity. For successful application of powder coatings on plastic substrates, surface resistivity of the substrate has to be less than 108 Ohm/Square (from our previous work published in European Coatings Journal, 20171). This places it in the conductive, static dissipative range as per Figure 1. (We define a successful application as uniform appearance, film formation and deposition of powder particles on substrate as well as in recess areas where there is no direct line of sight at the time of application.)

Quantification of classification of surface resistivity:

Anti-static
Decay rate (seconds to decay), 5000 to 50 V at 12 % relative humidity
Standard : MIL PRF 8705 D, NFPA 56A

Static dissipative (ESD)
Surface resistivity (ohm/square)
Surface resistance (Ohm)
Standard : ASTM D257, ESD STM11.11, IEC 60079-0

Conductive
Volume resistivity (Ohm-cm)
Surface resistivity (Ohm/square)
Standard : ASTM D257

EMI/RFI shielding
Shielding effectiveness (decibels of attenuation)
Standard : ASTM D4935

Figure 2: (Left) Uncoated polycarbonate/ABS composite; (Right) polycarbonate/ABS coated with UV curable powder coating.

We evaluated different types of conductivity agents such as quater- nary ammonium compounds (QAS), carbon black, graphene and also conductive nanoparticles. To determine the most suitable conductiv- ity agent, we formulated a design of experiments and coated various porous and non-porous, non-conductive substrates with different conductivity agents at various loadings. Their surface resistivities are given in Table 1 & Figure 5 & 6. These results show very little or no discrepancy since the variations on all of the substrates are very small.

Apart from conductivity imparted, using QAS has several disadvantag- es, since they are humidity-, process- and temperature-dependent. Their migratory nature does not ensure sufficient flexibility or adhe- sion of top coat to substrate. For conductive carbon black, significantly high loading is required to get low enough surface resistivity so that the powder coating forms a uniform film, and they deteriorate me- chanical properties of the film.

Table 1 also shows that if we load the conductive agent beyond a certain point then the decrease in surface resistivity is not necessarily substantial or linear. We need to find the optimum amount for porous substrates (such as MDF) and non-porous substrates (e.g. poly- carbonate, PC/ABS, glass and wood-plastic composite, which is less porous comparatively).

Figure 3: (Left) Uncoated wood-plastic composite (WPC); (Right) wood-plastic composite coated with UV curable powder coating.
Figure 4: Cohesive failure of powder coating on wood-plastic composite after Positest pull-off adhesion test.
Figure 5: Log of surface resistivity (Ohm/Square) Vs. % loading of conductivity agent on total formulation solids.

Adhering Coatings on Plastics

The coatings and ink industry faces the challenge of the adhesion of liquid and powder coatings to plastics, especially thermoplastic olefins. Conventional approaches such as flame treatment, corona discharge, gas plasma, UV exposure and chemical oxidation can be used to oxidise the surface of the substrate to promote adhesion. Oxidising the surface increases polar contributions to surface energy and produces more polar sites for bonding without altering the dispersive contribution significantly. The coating is best applied soon after treatment because the oxidation produces short-lived free radical species and is partially reversible. A major difficulty with ‘radiative’ techniques is achieving uniform surface coverage without over-treating, which introduces chain-scission and can lead to cohesive failure within the surface of the substrate.

The amount of halogen in the modified polymeric adhesion promo- tor determines the compatibility with various paint systems. Once the polymeric adhesion promoter is dispersed with conductive nanoparticles, it associates with plastics and composite substrates via dispersion interaction and adheres to it. Halogenated material and grafted functional groups add polarity to CAP which promotes interfacial adhesion to the substrate and the powder/liquid top coat.

Figure 6: Log of surface resistivity (Ohm/Square) vs. % loading of conductive nanoparticle on total formulation solids.
Figure 7: (Left) Uncoated curved porcelain tile; (Right) curved porcelain tile coated with low temperature cure powder coating with texture finish.
Table1:Surfaceresistivityofcoatedsubstrateusingdifferenttypesofconductivityagentatvariousloadings.
Table 2: Heat deflection temperature of different plastics and type of powder coating that can be used.

Type of Substrate Matters Greatly

Not all plastic substrates can withstand the high curing temperatures of conventional powder coating, 160-200 °C. Most plastics tend to soften, degrade or even melt at such high temperatures. It is safe to apply and cure powder coatings below a substrate’s heat deflection temperature. The heat deflection temperature is a measure of poly- mer’s ability to bear a given load at elevated temperatures.

Method

CAPs were applied on a PC/ABS and MDF, wood-plastic composite curved porcelain tile at 10-14 μm dry film thickness using an HVLP spray gun at 20 psi air pressure at the nozzle. They were dried/cured at ambient temperature for 3-5 minutes.

A UV curable smooth, white, epoxy powder coating (Figure 2 & 3) and low temperature cure textured black hybrid (epoxy/polyester) powder coating (Figure 7) were applied using an electrostatic spray gun on substrates coated with CAP. The UV curable powder was melted first at 120 °C for 3-4 minutes and then cured using a conveyorized UV oven with a medium-pressure H-bulb, and low temperature cure powder was cured at 130 °C for 5 minutes.

Positest pull-off adhesion tests were carried out to determine interfacial adhesion. Multiple adhesion tests with 20 mm dollies were carried out to determine the interface of the coating failure and the force/ area at which failure happens. The dry film thickness of the CAP and the cured powder coating were measured using Positector B100/ B200, an ultrasonic film thickness gauge (Table 3, Figure 4 & 8).

Where Are We Now?

  • CAPs ensure sufficient dissipation of negatively charged powder particles applied by electrostatic spray equipment and promote interfacial adhesion.
  • CAPs work more efficiently at lower film thickness on non-porous substrates. On porous substrates higher film thickness may be required since some of the material would be absorbed by a porous substrate.
  • CAPs enable successful application of powder coating on various plastic composites (uniformity, film formation, ability to coat recess areas, etc.).
  • CAPs can significantly increase transfer efficiency of applying liquid or powder coating to plastic composites having complex geometry.
Figure 8: Graphical representation of Positest Pull-off adhesion test, ASTM D4541 – cohesive failure within powder coating, no interfacial adhesive failure.
Table 3: ASTM D4541 Positest AT-A pull-off adhesion test.

3 questions to Atman Fozdar

How do you define “traditional“ and “non-traditional“ substrates?
Current electrostatic application technology only allows metallic substrates (which are inherently conductive and need to be grounded to dissipate static charge) to be successfully powder coated. Conductive Adhesion Promoter (CAP) technology enables successful application of powder coatings on non-conductive or non-traditional substrates like glass, ceramic, plastics, composites, wood, WPC etc. by making the surface of the substrate conductive and by improving interfacial adhesion between powder coating and the substrate, which was not possible otherwise with conventional Quaternary Ammonium Salts and other approaches.

What other plastic coating applications are feasible besides light vehicles applications?
Plastic composites used in automotive is just one of the examples to demonstrate CAP technology. Practically, any plastic substrate or composite (used in appliance, construction, medical or industrial areas) that can withstand 120 °C (melting temperature of powder) can be powder coated. We an- ticipate a huge potential for the wood-plastic composite market as well as recycled plastics for more efficient coating application with higher first-pass transfer efficiency and zero VOC.

What are the most important challenges that must be overcome before commercialisation of this technology?
We’re working on optimising formulations for porous substrates like MDF. In addition to that, a zero VOC water-borne version of CAP takes about 8-10 minutes to dry/cure. We’re evaluating other polymers to reduce dry/cure time so that it can be used in continuous/conveyorised environment for increased productivity.

References

[1] Fozdar A., Mannari V. “Development of Low VOC Static Dissipative Coat- ing for Powder Coating Non-Traditional Substrates.” European Coatings Journal, April 2017.

Featured Photo: Source: Dmitry Perov – stock.adobe.com

No Heat or Light? No Problem – with Curing Agents for Ambient Cure Coatings

Posted on by chemical dynamics

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.

Paint swatches on wall - learn how curing agents can improve performance in ambient cure coatings in the Prospector Knowledge Center.
Copyright: archidea / 123RF Stock Photo

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

Chemical reaction: Hexamethoxy methyl melamine + Polycarbonate -> Polyurethane

2. Polycarbonate + Polyamine ⇒ Polyurethane

Chemical reaction: polycarbonate + Polyamine -> Polyurethane

3. Polycarbamate + Polyaldehyde ⇒ Polyurethane

Chemical reaction: 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.

Chemical reaction: ambient cure moisture scavenger

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.
chemical reaction in ambient cure coatings - learn more in the Prospector Knowledge Center
  • 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 GroupResinCross-linkable GroupCross-linked group
PolyaziridineR-COOH(carboxyl)Acetyl urea
Silane Triethoxy silane and aliphatic epoxy Dual self-cure mechanismSiloxane & epoxy ester
Carbodiimide R-N=C=N-RR-COOHN-Acyl Urea
Isocyanate prepolymerR-NCOR-OH(hydroxyl) R-NH2(amino generated from reaction of water with isocyanate)Urethane  Urea
HydrazideR-C=OKetoneHydrazone

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