INVESTIGATION OF THE THERMAL ANALYSIS OF ORGANIC PIGMENTS CONTAINING A CARBOXYL GROUP

ABSTRACT

This article presents the synthesis of an organic pigment based on chelate compounds—metal phthalocyanine containing a carboxyl group. The obtained results were investigated based on thermogravimetric analysis data. The pigment synthesis was carried out by heating at high temperatures, and the study examined how thermal exposure affects its physicochemical properties, color intensity, and thermal stability. As the temperature increased, various stages of thermal decomposition of the organic pigment were identified, and the chemical components decomposed at each stage were determined. According to the analysis results, the synthesized pigment demonstrates sufficient thermal stability up to 800 °C, with a residual mass of approximately 48.71% at this temperature.

АННОТАЦИЯ

В данной статье представлен синтез органического пигмента на основе хелатных соединений - металлофталоцианина, содержащего карбоксильную группу. Полученные результаты были исследованы на основе данных термогравиметрического анализа. Синтез пигмента проводился методом нагревания при высокой температуре, и было изучено, как температурное воздействие влияет на его физико-химические свойства, интенсивность окраски и термическую стабильность. С повышением температуры были выявлены различные стадии термического разложения органического пигмента, при этом для каждой стадии определены химические компоненты, подвергшиеся распаду. Согласно результатам анализа, синтезированный пигмент обладает достаточной термической устойчивостью до 800 °C, при этом остаточная масса пигмента при данной температуре составила приблизительно 48,71 %.

Keywords: Phthalic anhydride, thermal analysis, metal salts, carboxyl group, urea, heat release, heat absorption.

Ключевые слова: Фталевый ангидрид, термический анализ, металлические соли, карбоксильная группа, мочевина, выделение тепла, поглощение тепла.

Introduction

More than 90% of the phthalocyanines produced worldwide (over 80,000 tons per year) are used as colorants—pigments and dyes [1]. Of this amount, approximately 40% are used in color printing inks, 30% in paints and coatings, 20% for coloring plastics, and 10% in other formulations [2]. Phthalocyanine derivatives account for approximately 25% of all commercially available synthetic organic pigments [3].

The high demand for phthalocyanines is due to a combination of critical properties required for pigments: vibrant color, high resistance to fading and weather conditions (lightfastness and weather fastness), resistance to solvents, acids, and alkalis, thermal stability, and cost-effectiveness (due to a high extinction coefficient). Moreover, the availability of starting compounds and the relatively simple synthesis technology of phthalocyanines contribute to their excellent cost-to-performance ratio.

According to the Colour Index, unsubstituted and partially chlorinated copper phthalocyanine blues are designated as Pigment Blue (P.B.) 15 (15:X, X = 1–6) with the number 74160; fully halogenated green pigments are classified as Pigment Green (P.G.) 7 C.I. 74260 (CuPc-Cl₁₄–₁₅) and P.G. 36 C.I. 74265 (CuPc-Cl₄–₈-Br₄–₁₂); the metal-free γ-H₂Pc is listed as P.B. 16 C.I. 74100.

The exact shade of a phthalocyanine pigment (Figure 1) depends on its crystalline form (ε- and α-modifications exhibit reddish tones, while β- and γ-modifications have greenish hues), as well as the number and type of halogen atoms present (P.G. 7 appears blue-green, while P.G. 36 has a yellowish-green tone) [4].

Figure 1. Shades of the main phthalocyanine pigments based on information about commercially available Heliogen pigments (BASF trademark).

The α-form of copper phthalocyanine (P.B.15:1), stabilized through partial chlorination, most used among blue phthalocyanines in plastic coloring and paint and coating formulations, including for achieving solid and metallic shades in automotive coatings. This form of CuPc exhibits excellent resistance to organic solvents, lightfastness and weather resistance, high thermal stability, and migration resistance in all media. It has strong tinting strength in polyolefins, and most binders containing P.B.15:1 meet all necessary requirements at temperatures up to 300 °C.

The unstable α-form of CuPc (P.B.15) is used to a limited extent in water-based paints due to its high coloring strength and is stable up to 200 °C. At higher temperatures and in the presence of aromatic solvents, it recrystallizes into the β-form.

The β-form of copper phthalocyanine (P.B.15:3), which is stable, is widely used in multicolor printing inks, for coloring plastics and rubbers, and in textile printing. Its pure turquoise hue is used as a standard blue color in offset and letterpress printing systems. Due to difficulties with dispersion, P.B.15:3 has limited application in polyolefins but shows high stability in plasticized polyvinyl chloride.

Flocculation-resistant pigments P.B.15:2 and P.B.15:4 are obtained by surface treatment of α-CuPc-Cl₀. ₅–₁ or β-CuPc using their water-soluble derivatives [5].

The ε-form of CuPc (P.B.15:6) has the most pronounced reddish hue of all blue phthalocyanine pigments and extremely high tinting strength. However, due to its significant instability, this crystalline modification is used only to a limited extent in the composition of color filters for LCD and TFT displays [6].

The unstable γ-form of metal-free phthalocyanine (P.B.16), which has a greener hue and higher tinting strength compared to β-CuPc, is also used to a limited extent, particularly in the creation of metallic paints.

All blue phthalocyanine pigments may cause warping of partially crystalline thermoplastics (such as polyethylene and polypropylene). To prevent this, CuPc with a higher degree of chlorination (CuPc-Cl₃) is used [7].

Green pigments P.G.7 and P.G.36 exhibit even higher lightfastness, weather resistance, and solvent resistance than blue phthalocyanines. However, due to their high molecular weight, they have lower coloring strength.

Their primary area of application is in paints and coatings, especially those used outdoors. The transparent (lassur) form of P.G.36, like P.B.15:6, is used in color filters for LCD displays [8-9].

Materials and Methods

In the synthesis of an organic pigment containing a carboxyl group, a heat-resistant glass vessel was initially charged with 30 g of urea and 18 g of phthalic anhydride, then heated in an oven at a temperature of 130–150 °C for 10–15 minutes. After that, 4 g of cobalt (II) chloride and 13.5 g of a metal carboxylate were added, and the reaction mixture was heated up to 190 °C. A catalyst was then added, and the mixture was stirred until a homogeneous mass was formed, turning into a viscous dark blue mass.

The resulting mass was calcined at 220–240 °C for 2 hours. After the reaction was completed, a bluish porous solid was obtained. The final product was cooled to room temperature and thoroughly ground in a mortar. Concentrated sulfuric acid (90%) was gradually added until a paste-like consistency was achieved. The resulting mixture was then thoroughly washed with distilled water to neutralize it. Once neutral, the solution was filtered using a Buchner funnel, and the obtained pigment was dried in a drying oven at 85 °C for 1–1.5 hours. The final product yield was 82.6% of the total mass.

Results and Discussion

Figure 2. Derivatogram of the organic pigment containing a carboxyl group

Table 1.

As the temperature increased, the substance exhibited the following stages of decomposition.

Stage 1: 138.52 °C – Evaporation of Water and Volatile Compounds

At this stage, hygroscopic water, adsorbed solvents, or low molecular weight organic residues in the sample evaporate.

Stage 2: 209.10 °C – Decomposition of Aza Groups

At this temperature, the peripheral groups of the phthalocyanine molecule or the weaker bonds around the central ring begin to break down.

Stage 3: 337.87 °C – Destabilization of the Conjugated Ring System

At this stage, the core aromatic ring system of the phthalocyanine starts to partially degrade.

Stage 4: 646.20 °C – Major Structural Decomposition

A large portion of the main carbon skeleton undergoes thermal decomposition at this stage.

Stage 5: 777.91 °C – Final Decomposition

At this high temperature, the remaining aromatic residues and carbon segments undergo final decomposition, leaving behind a solid residue.

Conclusion

This TGA analysis demonstrated that the phthalocyanine-based organic compound possesses high thermal stability up to 800 °C. The substance decomposes in stages, with the greatest mass loss occurring around 646 °C. The results confirm that this compound can be used as a pigment suitable for high-temperature applications.

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