Curing is a fundamental aspect in the coating process. Let's know this interesting analysis based on thermoset coatings.
by Eng. Orietta León*
A coating is a thin layer that is applied to a substrate to improve its appearance, to protect it from damage, or to prevent the substrate from interacting with other substances. Coatings that are mainly used to improve the appearance of a surface are called decorative coatings while those that provide functionality are called industrial coatings such as corrosion protective coatings.
Industrial coatings are applied on substrates such as metals, concrete, wood, paper, glass or composite materials, among others. These in turn are subdivided by industrial sectors: Transport (planes, trains and ships), infrastructure (buildings, bridges and pipelines), petrochemical, packaging, among others.
Also, they are classified according to the binder used as polymeric and non-polymeric materials. Polymer coatings are made up of thermoplastic coatings and thermoset coatings. Non-polymer coatings include metals, oxides, crystals and biofilms. Unlike thermoplastic coatings, thermoset coatings are formed in situ and applied in liquid or powder form.
However, for thermoset coatings they are made up of one or two compounds. Some additional components such as thinners, accelerators, pigments can be added to form the coating just before application. A one-component system relies on heat, moisture, oxygen, UV rays, and electron beam radiation to heal. However, the most commonly applied technique for liquid thermoset coatings is the mixing of two components, usually called resin and hardener.
It is important to note that despite all the progress made in the area of thermoset polymers, a single layer of coating or only one type of thermoset coating does not provide the three key attributes of thermoset coatings: adhesion to the substrate, barrier properties or low permeability to gases or liquids, and resistance to the environment to which the coating will be exposed. So it is a common practice to apply a first layer that ensures good adhesion to the substrate, an intermediate layer that provides the barrier properties, and a top layer to protect the intermediate layer from UV damage or give additional functionality such as antifoam. Multilayer coating systems are often a combination of two or more types of thermoset polymers, these can also include layers of thermoplastic materials.
Epoxy-based coatings occupy more than a quarter of all coatings consumed globally in industrial applications followed by polyurethanes, acrylics and alkyds. In the composition of powder coatings of epoxies and polyurethanes are included functional polyesters with carboxyl and hydroxyl groups, acrylics, silicones, fluoropolymers. So thermoset coatings have a wide range of adhesion, barrier and resistance properties that can be adapted to each substrate and service environment.
On the other hand, the chemistry and structure of the interface between the coating and the substrate are fundamental to achieve the required adhesion in all applications where thermoset coatings are used. The coating-substrate interface is developed from the moment the coating is applied in liquid or molten form until it reaches an infinite viscosity in the curing process. Proper adhesion between the coating and the substrate means that the interfacial region shows no cohesive or adhesive faults after the coating was applied and cured.
The adhesion force between the coating and the substrate is the result of a complex physical and chemical interaction, and therefore, significant effort is required not only to develop a thermoset system with excellent adhesion but also to ensure that the substrate surface is well prepared before the coating is applied.
To achieve good adhesion and barrier properties, the coating system must flow, moisten the substrate and cure before commissioning. The formation of films of epoxy coatings show a profile of curing isotherms, which includes the degree of conversion, reaction rate and viscosity of the coating as a function of time.
The kinetics of curing reaction initially depends on the type and nature of the functional groups and the amount of catalyst in the formulation. At this stage, the molar mass, viscosity and glass transition temperature (Tg) increase linearly with the degree of curing.
As the curing reaction proceeds, new functional groups are generated. In epoxy coatings, a hydroxyl group is generated each time an epoxy group is opened by a nucleophile or hardener. These hydroxyl groups can catalyze the curing reaction. Although the concentration of epoxy groups decreases during the reaction, the curing reaction rate increases due to the autocatalytic effect of the new hydroxyl groups produced during the curing reaction. In the self-cathalized region the molar mass and viscosity show a strong nonlinear dependence on the degree of curing.
However, the Tg increases linearly with the curing level. As the curing reaction progresses, cross-linking increases and macroscopic gels begin to form. The point at which the first macroscopic gel appears is called the gelling point. At this stage, the molar mass and viscosity of the coating approach infinity, and the material stops flowing for all practical purposes. At the point of gelling, the coating still contains a significant amount of soluble oligomers.
For almost all industrial applications, the Tg of the coating, in this region, is still significantly lower than the final curing Tg. As the curing reaction continues to progress, the Tg begins to approach the curing temperature, and the material begins to vitrify. Vitrification limits the mobility of molecule segments and their ability to diffuse into other molecules, and consequently, the curing reaction rate is limited. The use of solvents in the formulation of liquid coatings increases the vitrification point allowing a higher degree of curing at a given curing temperature. The type and amount of solvent are also critical to achieving proper film formation.
In addition to adjusting the chemorabilia of the coating to achieve good film formation in a reasonable time, the barrier properties of the coating or its low permeability to corrosive agents must also be adjusted to the atmospheric intent or immersion service by using a wide range of resins, hardeners, solvents, pavers and additives.
Finally, the chemical and physical nature of the surface to be coated is also fundamental to the intended application.
References consulted
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- F. Aguirre, A. Shafi, E. Trottier, et al., 2009. Rheo-kinetic modeling of fusion bonded epoxy powder coatings at standard and low application temperature. Eighteenth international conference on PIPELINE PROTECTION. Antwerpen. BEE. 4-6, pp. 81-91. Antwerp, Belgium: BHR Group, Cranfield.
- H.Q. Pham, M.J. Marks, 2006. Epoxy resins. In: Kirk-Othmer Encyclopedia of Chemical Technology. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
- A. Adomenas, K. Curran, M. Falconer-Flint, 1993. Epoxy resins and curing agents. In: Surface Coatings, Springer, Netherlands. pp. 179-192.
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- B. Ellis (Ed.), 1993. Chemistry and technology of epoxy resins. First ed. Blackie Academic & Professional. London, pp. 42-43.
- P.G. de Lange, 2004. Powder Coatings: Chemistry and Technology. Vincentz Network. Hannover, Germany. ISBN 9783878707844.
- U. Meier-Westhues, 2007. Polyurethanes: Coatings, Adhesives and Sealants. Vincentz Network. GmbH & Co KG. Hannover, Germany.
- M.F. Sonnenschein, 2015. Polyurethanes: Science, Technology, Markets, and Trends. Wiley, Hoboken, New Jersey, USA, p. 42.
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