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Coating Formulations > Organic Coatings > Flashcards

Organic Coatings Flashcards

1
Q

why produce pre-painted steel

A
  • Savings on effluent and solvent emissions
  • Energy savings
  • Press lubrication savings
  • Better control of quality
  • Added value
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2
Q

Colorcoat Line

A

1) De-coiler
2) Cleaning
3) Accumulator
4) Pre-treament
5) Primer Oven 400C
6) Quench
7) Finish Coater
8) Finish Oven
9) Emboss
10) Exit Accumulator
11) Inspect

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3
Q

Colorcoat components

A
  • Substrate
  • Sacrificial Zn coating
  • Pre-treatment
  • Pigment primer
  • Topcoat
  • Backcoat
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4
Q

Phosphate Conversion coatings (pre-treatments)

A
  • The anodic reaction replenishes the Zn2+ ions and the Cathodic reaction consumes the protons causing and increase in pH at the interface
  • The cathodic pH increases the drive for precipitation of insoluble zinc phosphate
  • Zn3(PO4)2
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5
Q

Pre-Treaments

A
  • Act as a secondary barrier
  • Provide a mechanical key for paint
  • Provide chemical adhesion promotion
  • Contain corrosion inhibitor species
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6
Q

Activation (phosphate)

A
  • Accelerates coating (phosphate deposition) rate and also suppresses hydrogen evolution
  • Activators are oxidising agents
  • H2O2, NO3-, ClO3-
  • Reduction of activator replaces hydrogen evolution as cathodic reaction
  • If Activator used, phosphating can be very rapid
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7
Q

Conditioning (phosphate)

A
  • Ensure metal phosphate crystallises on the metal surface and not in the bath
  • Strip washed with colloidal dispersion of ceramic particles (TiO2 or ZrO2) which are insoluble in water
  • These stick to metal surface and act as nuclei for the growth of the metal phosphate
  • Energy of the surface is higher than the bulk
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8
Q

Additional metal cations

A
  • Added to modify the coating

- Fe can be added to Zinc coatings to form phosphophyllite which provide better paint adhesion

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9
Q

Chromate conversion coatings (pre-treaments)

A
  • Chromating bath containing Na2Cr2O7 and H2SO4
  • Anodic reaction is metal zinc dissolution
  • Cathodic reaction is the reduction of dichromate
  • Cr2O7^2- and Cr3+ react together to precipitate an hydrated mix of Cr(VI)-Cr(III) oxide
  • Cr2O3.CrO3.nH2O(s)
  • If no activators present the anodic process quickly stops, very thin passivation coating
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10
Q

Activation (Chromate)

A
  • Aimed at preventing passivation
    1) Reducing agents (H2CO2), electrochemical oxidation of the activator replaces metal dissolution as anodic process
    2) Aggressive ions (Cl-, SO4^2-) which attack the metal surface preventing passivation and keep anodic dissolution going
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11
Q

Electrochemical acceleration

A

Instead of using oxidising or reducing agents as activators, additional current may be supplied to the metal strip

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12
Q

Possible alternative to chromate

A
  • Silane pre-treatment

- chromate is toxic but difficult to match wet adhesion resistance

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13
Q

Constituents of paint

A
  • Coloured pigment (Colour and opacity)
  • Corrosion inhibitor (Chemically inhibit)
  • Binder (Matrix to hold pigment in place)
  • Extender (Improve adhesion, strenghen film, save binder)
  • Solvent (VOC or water to reduce viscosity of paint for better application)
  • Plasticiser (modify properties of resin, lower Tg)
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14
Q

Reverse Roll coating

A
  • Coating material measured onto the applicator roller by precision setting of the gap between upper metering roller and application roller
  • Coating wiped off the roller by substrate as it passes around the support roller
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15
Q

Epoxy resins

A
  • Excellemt adhesion and physical resistance
  • High resistance to chemicals (corrosion)
  • Ether linkage is strong
  • Made via epoxy groups and substituted phenols
  • Size of polymer can be controlled to give viscous liquids to high MP solids
  • Susceptible to UV degredation, so excellent interior
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16
Q

Polyurethane resins

A
  • Made from a di-isocynate and polyol
  • Urethane linkage
  • Can be attacked by a base
  • If polyol had more than 2 hydroxyl groups, adjacent chains could could be linked together, which would make a stiffer structure
  • Limit on MW in paint
  • Could add linking groups as you apply paint
17
Q

Plastisol paints

A
  • Dont contain water
  • Mobile liquids
  • Contains PVC (polymer), a plasticiser, and a VOC solvent in small quantities
  • The VOC and Plasticiser must be full miscible
  • At the curing temp, the VOC solvent evaporates
  • The plasticiser then enters, swells and coalesces with the PVC
  • VOC must be a poor solvent for PVC
  • Plasticiser needs a greater affinity for the polymer than itself
18
Q

Slot die coating

A
  • The coating is squeezed out by gravity or under pressure through a slot and onto the substrate
  • Line speed is much faster than the speed of extrusion
  • This enables coatings to be considerably thinner than the width of the slot
  • Speed dictates thickness
19
Q

Powder coatings

A
  • use of fine powder (<1micron)
  • powder is sprayed low pressure jet
  • spray gun nozzle electrostatically polarised relative to workpiece
  • powder pick up charge and are attracted to work piece
  • stick electrostatically
  • heating oven cures coating by fusing particles
  • As solvent/liquid is bypassed, coating can be very tough and chemically resistant
20
Q

Nature of organic coatings

A
  • Good degree of corrosion protection by barrier properties
  • Very permeable to water and oxygen so do not effect the rate of underlying corrosion by being a barrier to water and oxygen
  • High electrical resistance, impedes movement of ions, reduces corrosion current to small value
21
Q

Good organic barrier criteria

A
  • Good wet/dry adhesion (strong adsorption of binder at metal-coating interface)
  • Low conductivity (Resistance inhibition, stop ion motion in film)
  • Chemical stability to its environment (OH-, UV, thermal)
22
Q

Cathodic Delamination (CD)

A
  • Occurs whenever metal becomes exposed to scratches
  • In CD cell, thin layer of electrolyte which penetrates beneath a region of already delaminated coating, acts to couple anodic metal dissolution (at coating defect) to cathodic oxygen reduction (at site of disbondment)
  • Alkaline env forms at the cathode
23
Q

4 region in delamination process in terms of Ecorr

A

1) Region immediately next to defect where Ecorr is low and similar to freely corroding metal
2) Region of slight Ecorr gradient resulting from ohmic resistance to ionic flow in thin under-coating electrolyte layer
3) Sharp Ecorr gradient - cathodic delamination front where coating disbondment takes place
4) Region of yet undelaminated polymer where O2 can diffuse to surface but no electrolyte and metal cannot corrode, Ecorr is high

24
Q

Kinetics of cathodic delamination

A 
Rapid (Unihibited) delamination:
-Xdel = Kd (tdel - ti)^1/2
-Kd increases with the aqueous diffuison coeffient of group 1 (Li < Na < K)
Slow (inhibited delamination:
-Xdel increases linearly with time
-Not effected by Kd
25
Q

Requirements for cathodic delamination

A
  • Group 1 metals must be present in corrosive electrolyte
  • Good electrocatalyst (Some metals better than others for the cathodic oxygen reduction)
  • Phases of good electrocatalysts must be continuous to advance
26
Q

Possible Contribute mechanism for disbondment

A
  • Alkaline dissolution of pre-existing oxide layer
  • Alkaline electrolyte displacing coating at substrate-coating interface
  • Alkaline electrolyte reacts with coating to break covalent bonds between substrate and coating
  • Short lived intermediates that may disrupt the bonding (H2O2, HO radicals)
27
Q

Filiform Corrosion (FFC)

A

.-When aggressive ions (Cl-) permeate through organic coating through a defect and accumulate as interface

  • Only occurs in presence of atmospheric O2
  • Propagate in relatively straight lines
  • Corrosion filaments consist of mobile electrolyte filled head containing metal cations and aggressive anions, and a tail of dry corrosion product
  • Low pH at leading head
  • Driving force is differential aeration arising from facile O2 transport through filament tail
  • Oxygen conc cell directs anodic metal dissolution at leading edge of filament head while principal site of cathodic oxygen reduction lies towards the trailing edge of the active head
28
Q

Mechanism of disbonding in FFC

A
  • Anodic undercutting of organic coating through metal dissolution
  • Significant hydrostatic pressure may produce mechanical disbondment
29
Q

Requirements for FFC

A
  • Agressive anions present in initiating electrolyte
  • Metal cations formed to be polyvalent and give rise to acidic pH through hydrolysis
  • Metal anion salt to be hygroscopic to maintain head electrolyte droplet at relative humidity
  • Metal should not form corrosion products with the aggressive anions necessary for FCC
30
Q

Corrosion Inhibitor Pigments

A
  • Powders and dispersed in priming paint
  • Mostly “sparingly soluble salts”
  • Combination of metal ions and anions such as phosphorus derived and chromate
  • Although chromate and lead are very effective inhibitors, their very limited due to environmental and toxicological regulations
31
Q

Problems with “sparingly soluble inhibitors”

A
  • Inhibitor can be leached by water flowing over coating
  • Solubility dependent on chemical composition and particle size
  • If inhibitor salts are too water soluble osmotic blistering can be the results
32
Q

Ion Exchange Pigments

A
  • Completely insoluble in water but charged
  • Counter charge carried by the inhibitor ions which are exchangeable
  • Ion exchange pigments are immune to osmotic blistering
  • Inhibitor is only released when electrolyte is present (smart release)
33
Q

Cation Exchange

A
  • Calcium Shieldex
  • Particles composed of hydrated silica, washed with Ca(OH)2 to produce a negative surface charge
  • Exchangeable Ca2+ acts as cathodic inhibitor
34
Q

Anion Exchange

A
  • Hydrotalcite
  • Anionic clay where particles have a fixed positive layer charge.
  • Countercharge is provide by exchangeable carbonate anions
  • Used to scavenge Cl- and prevent filiform corrosion

Coating Formulations (2 decks)

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