Category: Case Studies

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, Chemical Dynamics LLC, USA.

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.

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.

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.

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-https://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.

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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.

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

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).

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.

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.

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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.

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