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Polymer Systems and Film Formation Mechanisms in High Solids, Powder, and UV Cure
Systems
J. Baghdachi, Ph.D.Coatings Research InstituteEastern Michigan University
(734) [email protected]
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Film Formation in Coatings
Outline:
• Thermoplastic systems• Thermosetting systems• Variables controlling property development • Stages of property development• Waterborne• Energy cure• Powder coating
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Almost all desirable properties of a coating film strongly depend upon the quality andintegrity of the coating film which in turn depends upon the polymer chemistry, formulation variables, the Tg of the dry film and the surface characteristics of the substrateamong other factors.
Film Formation
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Coating properties influenced by film formation process
AdhesionChemical resistanceMechanical propertiesDirt pick upFlowGlossPop (blister)SagSurface dry and hardness development
Film Formation
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Film Formation
Three major classification of film formers
• Thermoplastic materials • Chemically converting systems• Latex systems
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Film Formation
Chemical and Physical Variables Controlling Property Development
Molecular weightCrosslink DensityGlass Transition TemperatureBuilding blocksViscosityCure TemperatureFormulation variables
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Property Thermoplastic Thermoset
MW prior to application High LowMW as dry film High Very highCrosslink density Very low Moderate-v.highHardness Poor-good Moderate-ExcellentSolvent resistance Poor-good ExcellentChemical resistance Poor-Excellent Moderate-ExcellentPermeability, H2O Very low-high Very lowGloss Low-high High-very highRecoatability Excellent Poor-goodLow VOC use Poor Excellent
Film Formation
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Some Milestones:
• A Film is dry-to-touch when viscosity is >103 Pa.s
• To resist blocking for 2s at 20 Psi, need viscosity>107 Pa.s
• To serve as industrial enamel, viscosity often must be >1012 Pa.s, the viscosity at Tg.
Film Formation
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Film Formation
ln? = 27.6 -40.2 (T-Tg)
51.6 + (T-Tg)
Williams, Landel and Ferry (WLF) Equation
T, in Kelvin-degrees
Relationship between viscosity and temperature
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Thermoplastic Systems
Linear and Soluble Branched and Soluble
Polymer chains are longer but remain separate. Though they may coil around one another and exhibit branching, there is no primary valency bonding between chains.
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Film Formation by Evaporation of Solventfrom Solutions of Polymers-- Lacquers
• To have good film properties, polymer molecules must be large (very high molecular weight)
• Concentrated solutions of high molecular weight polymers are too viscous. Dilute solutions have high VOC.
Thermoplastic Coatings
However,
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Film Formation--Air dry (Lacquer)
DryFilm
High PolymerSolution
SolventEvaporation
Aging
Drying
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Film Formation--Air dry (Lacquer)
DryFilmAging
The cross chain linkages are weak secondary valency bonds broken relatively
easily as the film is dissolved or melted
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Principles of Film Formation
Stages of film formation (High Tg)
• At the early stages of drying, the rate of solvent evaporation is essentially independent of the presenceof polymer
• The rate of evaporation depends upon
-The vapor pressure -The ratio of surface area to volume of the film
-The rate of air flow
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Principles of Film Formation
Stages of Film formation
As the viscosity and Tg increase, free volume decreases, and the rate of solvent evaporation depends on how rapidly the solvent molecules can reach the surface of the film.
The rate of solvent loss is controlled by the rate of DIFFUSION of the solvent through the film.
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Principles of Film Formation
Lower Tg and more free-volume available
Solvent evaporates easily
Vapor pressure controlled
Higher Tg, and viscosityLower free volume
Diffusion controlled
Drying
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Film Formulation-Thermoplastic Polymers
The rate of solvent diffusion and evaporation also depends on the solvent structure and the solvent polymer interaction.
CH3COOCH2(CH2)2CH3 CH3COOCH2CH(CH3)2
n-Butyl acetate Isobutyl acetateDiffuses more rapidly Higher evaporation rate
CH3 (CH2)6CH3 CH3 (CH2)5CHCH3
n-Octane Isooctane
CH3
CH3
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Thermoset Systems Chemically reactive systems
Main Resin Cross-linkerPolyols Amino ResinsPolyols (Poly)isocyanatesPolycarboxylic acids EpoxiesPolycarboxylic acids CarbodiimidesPolycarboxylic acids Aziridines(Poly)amines, amides EpoxiesAcetoacetate (active methylene) Amino resinsAcetoacetate (active methylene) (Poly)isocyanatePolyols and polycarboxylic acids Siloxanes
Miscellaneous Combinations
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Thermoset Systems
• The mechanical properties of the film depend strongly
upon the Tg of the crosslinked polymer and upon the
degree of crosslinking
• Physical properties such as water and oxygen permeability, solvent and chemical resistance are affected by the degree of crosslink density.
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A number of extreme changes accompany crosslinking
Soluble InsolubleFlow Severely reduced flowGlass Transition Temperature Increase in Tg
Crosslink Terminology
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Crosslinking
Linear Polymer Crosslinker
Network (Crosslinked Polymer)
+
Crosslinking
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Thermoset Systems
Tg increases during crosslinking for three reasons:
• Chain mobility near crosslinks is constrained
• Crosslinkers are converted from plasticizers to network chain segments
• Mn of main resin increases sharply
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Thermoset Systems
Crosslinking requires that the reactants diffuse into a reaction volume
Small molecules may diffuse more readily than functional groups on a polymer chain
Water plasticizes coatings, lowering their Tg
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Thermoset Systems
If the diffusion rate is greater than the reaction rate, the reaction will be kinetically controlled
If the diffusion rate is slow compared to the reaction rate, the rate is mobility controlled
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Thermoset Systems
The major factor controlling rate is the availability of free volume
The free volume is large if the reaction temperature is higher than the Tg
If the reaction temperature is below Tg, the free volume is limited
At intermediate temperatures, the reaction is controlled by the rate of diffusion (mobility of reactants).
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Thermoset Systems
If the reaction temperature is above the Tg of fully reacted polymer, there will be no mobility effect
In ambient curing coatings if the Tg of the fully cured polymer is above the ambient, the reaction will become mobility controlled
As Tg approaches the cure temperature, reaction becomes slower
When Tg equals T, reactions become very slow and Vitrification occurs.
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Waterborne
Film Formation in Latex Systems
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Film Formation--Waterborne Coatings
Core-Shell Latex
Multiphase Latex
Dispersion
HOOCNEpoxy
COOH
OHNCOEpoxy
COOH
EpoxyAlkydAcrylicPolyesterPolyurethane
Resins
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Occurs in three overlapping stages
• Evaporation of water and co-solvents leading to close packed layers of latex particles
• Deformation of the particles from their spherical shape leading to a more or less continuous layer
• Coalescence, a relatively slow process in which the polymer particles and molecules interdiffuse across the particle boundaries and tangle, strengthening the film.
Film Formation from Latex
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What Derives Deformation?
• Capillary forces? • Powerful (up to 5000 psi) but short lived• Surface energy reduction?• Much weaker but longer lived
Film Formation from Latex
)
((.
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What Derives Deformation?
Tg of the latex particles is an important factor
• Tg of the outer shell is what counts• particles and film can be plasticized by water and
coalescent agents (solvents)
Film Formation from Latex
)
((.
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Film Formation
Top view of the drying latex showing three contacting particles with a capillary full of water in between
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Film Formation
Schematic representation of two spheres in contact after partial evaporation of water
Thin layer of water around the particles
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Coalescence
For a given latex, the lowest temperature at which coalescence occurs sufficiently to form a continuouscohesive film is called its
Minimum Film Formation Temperature MM FT (MFT)
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Film Formation--Latex
• Complete coalescence is a slow process
• The rate is affected by (T-Tg)
• Coalescing agents reduce Tg and MFFT.
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Coalescence
• Coalescence occurs as molecules interdiffuse. The distance the molecules travel to interdiffuse is considerably less than the diameter of a typical latex particle
• The rate of interdiffusion is directly related to T-Tg. As a broad rule, coalescence will not occur unless the temperature is at least slightly higher than Tg.
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Film Formation
Scanning Electron Micrograph of films prepared from a latex polymer
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Latex Film Formation
Stage IWater Evaporation
Stage IIParticles Deform
Stage IIICoalescence (aging)
Aqueous Latex
Close-Contact Particles
Packing of Deformed Particles
Mechanically Coherent Dry Film
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Crosslinking Latex
PolymerDiffusion
Fully diffusedFast crosslinking
Weak Boundaries
Weak film interfacial fracture Strong film cohesive fracture
Crosslinked film
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Film Formation in Energy Cured Coatings
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“Energy ” Curing
• “Energy” Curing - initiation by:– UV: 200 - 400nm light– Visible: typically 380 - 450 – Electron beam
• Radical mechanism• Cationic mechanism
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. +
O
O
O
ORR
.
. ..O
O+
M
O
ORR
OO
O
O M
Radical Curing Mechanism
Initiation Step
Propagation
Termination StepsM M
M RRM.
M..
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Cationic Curing MechanismCATIONIC CURING - UV
Initiation (Light & Heat)
O
R H+MF6-
O
R
H+
O
R
O
R
R
HO
+
initiation
hν
photoinitiator
HO
R+
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Common Photoinitiators
Photocleavage - Type I
2-hydroxy-2-methyl-1-phenyl-propan-1-one
1-hydroxycyclohexyl phenyl ketone
C CO
OHCH3
CH3
CO
OH
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+S+ S S+ X-X- S+ SX-
X = PF6- or SbF6
- counterion
Sulphonium Salt Cationic Photoinitiators
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bisphenol A diglycidyl ether diacrylate
Oligomers
CH2 CH C O CH2 CH CH2 O CCH3
CH3O CH2 CH CH2 O C CH CH2
OOHOHO
aliphatic urethane diacrylate
CH2 CH C O R O C NH CH2 NHO
CO
O R'
CH3 CH3
OCH3
C NHOO
CH2
CH3
NHCH3
C
CH3
RO
O C CHO
CH2
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Acrylated Acrylic
Oligomers
CH2 CH CH2 CH CH2 CH
C
O
R
O C
O
CH2
CH
CH2
OH
O
C
CH
CH2
O
O C
O
R'
O n
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Film Formation in Powder Coatings
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Film Formation in Powder Coatings
There are two major classifications of powder coatings,Thermoplastic and Thermosetting.
Thermoplastic powders melt and flow with the application of heat.
PVC, Nylon, Polypropylene, Vinyl, and Fluorinated resins
Thermosetting powders
Epoxy, polyester, polyurethane, and acrylic and combinations thereof.
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A finely ground mixture of ingredients in a resinous base, which are solid at
the time of application, but melt, flow, and coalesce
into a protective film in the presence of heat.
Substrate
Substratemelting
leveling/curing Substrate
POWDER COATING
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. +
O
O
O
ORR
.
. ..O
O+
M
O
ORR
OO
O
O M
Radical Curing Powder Coating
M M
M RRM .
M..
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UV Cured Powder
• These powders follow a combination of conventional and UV cured systems.
• The melt and low is accomplished by a brief exposure to a heat source
• The cure and cross linking is achieved by a brief exposure to an ultraviolet light source.
This technology is suitable for powder coating temperature sensitive materials such as wood products (MDF)
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Challenges
• Highly pigmented systems are slow to cure
• Film thickness limitations
• Balance of good flow and cure is critical
• Line-of-Site
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Film Formation
Substrate Substrate
Melt and flowApplied powder
IR
Finished wood
UV (395-445 nm) 2000-4000 mJ/cm2
Or other source
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Film Formation in Coatings
ReferencesOrganic Coatings: Science and Technology, Z., Wicks, F., Jones,
P. Pappas, John Wiley and Sons, 1999.
Protective Coatings: Fundamentals of Chemistry and Composition,
C. H. Hare, Technology Publishing Company, 1994.
Adhesion Aspects of Polymeric Coatings, J. Baghdachi, FSCT, 1997
Waterborne Coatings: A compilation of Papers from the Journal of
Coatings Technology, 2001
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Thank You!