Synthesis, Characterization of Bio-based Polyol and Assess the Effectiveness of Bio-based Polyurethane Direct-to-metal Coating System

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Synthesis, Characterization of Bio-based Polyol and Assess the Effectiveness of Bio-based Polyurethane Direct-to-metal Coating System

S. Sameera D. Mendis
Paints and General Industries Limited-Manufacturer of AkzoNobel Paints, Sri Lanka.
Received: 26 May 2023; Revised: 16 June 2023; Accepted: 22 June 2023; Published: 23 July 2023

Abstract – Crude oil is neither a long-lasting energy source nor a raw material source, has a high consumption rate relative to a low regeneration rate and creates massive environmental disorders. Polyurethane is well known and is the most popular film forming material in the coating industry because of its better performance. A coconut oil-based polyol (biobased polyol) was synthesized and acid value, viscosity, reaction water release, oil length, FTIR spectrum, differential scanning calorimetry and colourimetric index were assessed during the synthesis. A series of pigmented wet paint samples were prepared by bio-based polyol and the optimum paint sample was selected among them based on drying time, pencil hardness, dry film thickness, cross hatch test and cylindrical mandrel blending. Two wet PU paint samples, representing commercially available crude oil-based polyols, were prepared to compare in the same aspects with a PU paint sample made with a bio-based polyol (BBP).
According to the results, it was proven that bio-based PU paint showed equal magnitudes in film flexibility due to the cylindrical mandrel bending test, film hardness due to the pencil hardness test, substrate adhesion due to the crosshatch test and impact resistance. In some properties, bio-based PU exceeded at least one crude oil-based PU such as gloss, hard drying time and density. Meanwhile, bio-based PU systems must be improved in viscosity and water resistance to compete with artificial PU systems.

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Key words – Coconut oil, bio-based PU, nitrocellulose, surface coating.

I. Introduction

Generally, paints are made out of pigment, binder, solvent, and additives. Pigment provides colour and hiding power. A binder or resin gives adhesion to the surface. Solvent acts as a carrier for binder and pigments. Additives can enhance some properties, such as sag resistance, mold resistance, UV resistance, brushability, drying and more [1]. Industrial coating applications follow the three-coat system, which is mainly made out of polymeric materials. Ground coat is considered as primer, but many varieties are available as zinc chromate primer, zinc phosphate primer and 2K epoxy primer in the market. The other two components are base coat paint and clear coat. The clear coat is the outermost layer among them [2]. In addition to these layers, commercial vehicles have an extra coat, which is the lowest one and directly adheres to the metal and is called electrocoat. Electrocoat is an epoxy derived polymeric material deposited by the electrophoresis method. Acrylic resin systems are highly accepted for the manufacture of clearcoat and basecoat layers. Except for electrocoat, all the other coats are applied by spraying methods [3].
Direct-to-metal (DTM) coating is a system that applies directly to the metal substrate without a primer or basecoat. Therefore, it requires better adhesion, anticorrosive properties, weather performance and aesthetic appearance by itself, as in a multilayered system. Substrate wetting directly affects adhesion and it needs a well-balanced solvent combination to reduce surface tension. The most common binders in the DTM are epoxies, acrylics, polyurethanes, polyalkyds, and polysiloxanes. However, this single layer coating saves time on a painting job and reduces costs effectively [4]. Corrosion resistance is a very important factor that increases the life span of metal structures and reduces investment of structure maintenance costs. According to a study, the maintenance cost of direct damages from corrosion in the United States was 3.45% of GDP in 2016 [5].
When corrosion forms, both cathodic and anodic reactions take place on the same steel substrate, as shown below.