Elsevier

Progress in Organic Coatings

Volume 78, January 2015, Pages 239-243
Progress in Organic Coatings

Determination of degradation kinetics of two polyester thermosetting powder coatings using TGA and colorimetric analysis

https://doi.org/10.1016/j.porgcoat.2014.08.015Get rights and content

Highlights

  • Thermal stability of durable and superdurable powder coatings was investigated.

  • Activation energies were determined by applying Kissinger equation on TGA data.

  • Activation energy is about 25 kJ mol−1 higher for the superdurable sample.

  • Comparable results were obtained from colorimetric experiments on aged samples.

Abstract

Thermal stability and thermal degradation kinetics of two different polyester thermosetting powder coatings used as decorative finish in outdoor architectural applications, one standard durable and one superdurable, were investigated. The activation energies of the main stages of the degradation processes were determined by applying Kissinger's and Ozawa's equations on TGA data, which were obtained by using a thermobalance at different rates of heating in a mixed atmosphere of oxygen and nitrogen. The superdurable sample has an activation energy higher than that of the standard durable coating. Both the samples showed greater stability with respect to carboxylated polyesters with TGIC as cross-link agent and epoxy resins with dicyandiamide as reticulating agent. The results were compared to those obtained from aging experiment carried out in a oven at three different isothermal temperatures. The activation energy values were obtained from colorimetric measurements of the aged samples. Data from different approaches resulted closely comparable and it was possible to estimate the activation energies of the degradation processes from total chromatic changes with an error lower than 5% with respect to TGA. This last result highlights the possibility of obtaining kinetic data on these coatings also by using the colorimetric technique.

Introduction

Thermosetting powder coatings have a very important rule in the varnish industry because they guarantee, after curing, a cross-linked structure that provides a high-quality, durable finish. These powders are made by a mix of pigments, additives and fillers in a particular binder (reactive system) suitable to create a coating film. The method to manufacture electrostatic coating powders is melt mixing of the raw materials using a compounding extruder, followed by a fine grinding of the extrudate (average particle size of 30–40 μm). Once applied (with special guns) on the metal substrate and put in a oven at 180 °C for 20 min, the powder starts to softening. Contemporary, the reaction between resins and hardener initiates and permits the formation of the final film with a cross-linked structure. The success of powder coatings in the last decades was originated from the attractive combination of features that they offer: ecological safety, cost effectiveness, energy saving and excellent product performance. Powder coating materials contain no solvent, the process emits negligible, if any, volatile organic compounds (VOCs) into the atmosphere, the application plant is facile and economical [1], [2], [3], [4]. The families of resins of major actual importance for powder coating are polyesters, epoxy resins and the so-called hybrid resins, formed by the reaction of a polyester resin with an epoxy one [5], [6]. The choice of the resin is of paramount importance, because most of properties of the final coatings, mechanical in particular, are determined by the binder system, where the resin is the mayor component [7], [8], [9]. Powder coated finishes resist to scratches, corrosion, abrasion, chemicals, detergents and can be prepared with a wide range of colors and glosses both for interior and exterior applications like architectural, automotive, household appliances, fences, furniture, agricultural machines, and so on [10]. For outdoor applications the various types of powder coatings differ considerably in different geographical locations. The three major market areas in the world, Europe, USA and Japan, have in fact developed completely different systems, this due to historical reasons and availability of raw materials. In the USA polyurethanes with aliphatic polyisocyanates as cross linkers have dominated for many years, while in Japan powder coatings with acrylic resins as binders are market leaders [2]. In EU the systems commonly used for outdoor applications are based on polyester carboxylated resins and the technical specifications for this market are defined by the GSB and Qualicoat organizations.

Polyester-based powder coatings can be divided in standard (durable) and superdurable on the basis of UV resistance (3–4 time higher in the superdurable coatings). The standard polyester powder coatings are based on resins manufactured mainly using terephthalic acid (TPA) as principal diacid, meanwhile superdurable polyester powder coatings mainly contain isophthalic acid (IPA) as diacid. Unfortunately, the replacement of TPA with IPA provides lower flexibility to the final system [11], [12], [13].

A number of predictive studies is reported, together with time-consuming standardized procedures, for the study of durability, but some of these have been called into question in view of their largely unreliable results [14], [15]. Moreover, most of TGA analyses actually reported in the literature are limited to quite old-type and sometimes potentially toxic coatings, in particular carboxylated polyesters with TGIC as cross-link agent and epoxy resins with dicyandiamide as reticulating agent [16], [17]. The degradation of more recent coatings has been studied quite exclusively by DSC [13], [18], [19] and the use of other techniques, for example IR spectroscopy, is very limited [12]. In this paper we report a study concerning the degradation stability of typical polyester coatings used as surface and structure decorative finish in external architecture. In particular, we compared the activation energies for the degradation processes of two different kinds of coating, a standard durable and a superdurable. The kinetic parameters, determined by conducing the degradations with a thermobalance at different heating rates (TGA analysis), resulted comparable to those obtained from simple colorimetric measurements on thermally aged samples.

Section snippets

Materials

The polyester thermosetting coatings used in this study are available on market and were supplied by the manufacturer AkzoNobel Coatings S.p.A. as typical finish for external architecture surface. In the standard durable sample, named A1 in the paper, the main diacid is terephthalic acid (TPA), while in the superdurable powder coatings, named D3, the main diacid is the isophtalic one (IPA). The binder system is based in both the cases on polyester resins (EW 2500 – Acid Number 25–30 mg/KOH) and

TGA analysis

The study of thermal stability by means of TGA allowed us to analyze the degradation response of the samples [16], [23], [24]. For each powder coating the TGA runs were carried out at different rates of heating (from 5 to 20 °C min−1) in a mixed atmosphere of nitrogen and oxygen. It is to be noted that Morancho et al. [16] observed that the degradation processes of comparable compounds are very similar under oxygen, nitrogen and air. TG signals gave the weight losses W of the samples in function

Conclusion

TGA experiments showed three stages of weight loss for both standard durable and superdurable thermosetting powder coatings. The main reaction stage is associated to the degradation of the resin fractions. The higher stability of the superdurable powder coatings is highlighted by an increment of about 15 °C of the temperature corresponding to the maximum rate of weight loss. The activation energies coming from TG analysis were 214 kJ mol−1 and 251 kJ mol−1 for the standard durable and the

Acknowledgments

The authors gratefully acknowledgments the Cà Foscari University of Venice for the financial support (Ateneo Found 2013) and the AkzoNobel Coatings (Vicenza, Italy) for providing the samples.

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