Preferential interactions in pigmented, polymer blends – C.I. Pigment Blue 15:4 and C.I. Pigment Red 122 – as used in a poly(carbonate)–poly(butylene terephthalate) polymer blend

Abstract

Some important characteristics of selected pigments have been evaluated, using the inverse gas chromatography (IGC) technique, that indicate the occurrence of preferential interactions in pigmented polymer blends. Attention has been given to copper phthalocyanine pigments and to quinacridone pigments incorporated in polycarbonate–poly(butylene terephthalate) blends. Selected supporting techniques were used to provide supplementary information concerning the pigments of interest, C.I. Pigment Blue 15:4 and C.I. Pigment Red 122. For C.I. Pigment Red 122 and for C.I. Pigment Blue, the dispersive component of the surface free energy decreases as the temperature increases, indicating the relative ease with which the molecules can be removed from the surface.

Introduction

High-performance pigments are those with all-round properties that may be used in more specialised and demanding applications such as automotive paints and construction plastics [1], [2], [3], [4], [5], [6]. As polymeric and composite materials are replacing metals in applications such as automotives manufacture, demand for these high performance types of pigments has increased. Two such high-performance pigments, a copper phthalocyanine and a quinacridone, have been selected for use in an impact-modified blend of a polycarbonate (PC) and poly(butylene terephthalate) (PBT). It was considered to be essential that the pigments were characterised before they were added to the polymer formulations. This paper focuses on some of the more relevant pigment characteristics that were thought to influence the final pigmented polymer product.

Pigmented polymer blends are complex in several ways. They possess compositional heterogeneity as well as different extents and types of interaction, as seen in preferential adsorption behaviour [3]. The inverse gas chromatographic technique provides a means of quantifying the overall effect of such interactions as well as a means of studying features arising from the more specific interactions.

With respect to PBT, Santos et al. have studied the surface characteristic by means of the IGC technique. In their work the specific component of the adsorption of polar probes was noted to be endothermic. The change in the entropy of the system was positive, an uncommon situation in the case of IGC studies. The results were interpreted in terms of cleavage of H-bonds in the PBT and the formation of H-bonds between the probe molecules and the polymer. The Lewis acidity constant and the Lewis basicity constant correlated well with analyses of the repeating unit and the end groups of the polymer [7].

In an extension of their work, Santos et al., used the IGC technique to study the interactions between pigments (particularly cobalt aluminate) in impact modified PC-PBT blends. The determined values of the various types of interaction allowed the authors to provide a rationale for an interpretation of the phase separation and the phase preferences that exist in the chosen polymer blend system. An attempt was made to explain observed changes that took place in the physical properties and in the chemical properties of the pigmented polymer blends in terms of the data that were provided by the IGC evaluations [8].

Various other workers have considered the use of IGC in the study of pigmented polymer blends, with some emphasis being placed on colour development, gloss, rheological properties, adhesion and mechanical properties. Schreiber used IGC techniques in a study of pigmented, plasticized PVC systems, correlating IGC results with those concerning the rheological properties and the mechanical properties [9]. Lee et al., studied the significance of the acid–base properties of surface modified TiO₂ pigmentary particles and their influence on the dispersion qualities of the pigments when in polymeric matrixes [10]. Ziani et al., studied the dispersion stability of pigments in paint formulations and concluded that a correlation exists between the ease of dispersion and the acid–base interaction parameters, as determined by IGC [11]. Such findings support those made by Kunaver et al., in their IGC study of interactions in pigmented, high solids coatings formulations [12]. A summary of such work has been provided by Santos. However, whilst considering organic pigmented polymeric composites, Santos was mainly interested in interactions that take place between pigments and their polymeric matrix, as encountered in blend compositions [13].

The copper phthalocyanine pigment evaluated in this work was C.I. Pigment Blue 15:4. This is the flocculation-stabilised, β-phase form of the pigment. This pigment is bright, has a high tinctorial strength, has renowned durability and is very cost effective in use. There are two major methods of manufacturing copper phthalocyanine pigments. These methods are well described in the literature, as are the properties of the pigment [14], [15], [16].

The quinacridone pigment that was evaluated in this work was C.I. Pigment Red 122, a 2,9-dimethylquinacridone, a linear trans-quinacridone. It has a very clean, bluish shade of red. This quinacridone has excellent migration fastness and heat stability [17], [18], [19], [20], [21], [22], [23], [24], [25].