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Thermal characterization of solvent-free epoxy coatings by rheology and kinetics combined

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Abstract

Rheology and kinetics are a pair of indispensable attributes in defining thermally reactive polymer coatings and paints. In this work, cure paths of volatile organic compound-free epoxy powder coatings, under heat, were characterized by viscoelastic and kinetic techniques. The essential rheological meanings of the gel point, the minimum complex viscosity, and the shear moduli from dynamic mechanical analysis curves were elucidated from applied formulation perspectives. Meanwhile, the maximum cure reaction rate and the peak temperature from differential scanning calorimetry exothermic profiles were quantitatively interpreted using primarily the extent of cure along with viscoelastic data. Vitrification and devitrification during cure were observed for high glass transition (Tg) epoxy formulations, unless a fast rate of heating was employed to a high enough temperature. Thorough and complete cure description provides strong insights for product development, allowing flow and cure properties to be balanced for optimal line performance. The consolidated approach is applicable to nearly all B-staged thermosetting systems of 100% solids including conditioned prepregs and a variety of adhesives per applications, as well as direct-to-metal primers or topcoats per functions.

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Abbreviations

A :

Pre-exponential factor in cure kinetics

E a :

Activation energy (J/mol)

m, n :

Reaction orders in autocatalytic model

α :

Extent (or degree) of cure (%)

\(\dot{\alpha }\) :

Cure reaction rate, = dα/dt

α gel :

Extent of cure at gelation

\(\dot{\alpha }_{ \hbox{max} }\) :

Maximum cure reaction rate, = dαmax/dt

β :

Rate of heating for cure, or heat rate (°C/min)

δ :

Phase angle

ΔH :

Enthalpy, heat of reactions, reactivity (J/g)

γ :

Strain (%)

\(\dot{\gamma }\) :

Strain rate (1/s)

K :

Coefficient in Power law

λ :

Relaxation time (s)

De :

Deborah number

E: E′, E″:

Tensile modulus: storage, loss (or elastic, viscous component)

η :

Shear viscosity (Pa s)

η e :

Extensional viscosity

η*:

Complex or dynamic viscosity

η*min, η*max :

Minimum viscosity, maximum viscosity

G: G′, G″:

Shear modulus: storage, loss (or elastic, viscous component)

G′min, G″min :

Minimum storage, loss modulus (Pa)

G′max, G″max :

Maximum storage, loss modulus

R :

Gas constant (8.314 J/mol K)

T :

Temperature (°C or K)

t :

Time (s)

T cure :

Cure temperature

T gel :

Temperature at gelation

Tan(δ):

= G″/G′

T g :

Glass transition temperature (°C)

T peak :

Peak temperature on DSC exothermic curves

τ :

Shear stress (Pa)

Type 4, 5, 7 epoxy:

Repeating units of 5, 8, and 11, respectively, in BPA-derived resins

υ :

The Poisson ratio

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Acknowledgments

The author greatly thanks Robert G. Polance, a Senior Staff R&D Chemist at the Sherwin-Williams Company in Minneapolis, the USA, for his DMA operations and valuable perspectives in support of this work.

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  1. Sherwin-Williams Company, 1101 South Third Street, Minneapolis, MN, 55415, USA

    Weih Q. Lee

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Lee, W.Q. Thermal characterization of solvent-free epoxy coatings by rheology and kinetics combined. J Coat Technol Res 17, 145–155 (2020). https://doi.org/10.1007/s11998-019-00248-9

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