Synthesis and properties of some polycyclic barbiturate pigments

Abstract

The synthesis, characterisation and properties of some polycyclic barbiturate pigments are described. The pigments have been tested as colourants for plastic and paint and the results of lightfastness, heatfastness and contact bleed tests are reported. Results suggest that only the barbiturate pigments with large aromatic moieties such as the pyren-1-yl and fluoren-2-yl substituents have good lightfastness properties at reduced shades in plastic and only the pyren-1-yl pigment has good lightfastness properties at reduced and full shades in paint. However, all the pigments suffer from poor heatfastness properties in plastic. The absorption coefficient (ε) of some of the pigments is low confirming their weakness in colour strength when tested in high density polyethylene.

Introduction

The uses of barbituric acid derivatives are many and varied. In the past, they have been widely used in medical applications as sedative, hypnotic and local anaesthetic drugs [1], [2], [3], [4], [5]. However, in recent years, they have been studied as antitumour [6], anticancer [7], [8] and anti-osteoporosis agents [9]. They have also been used as disperse dyes with strong fluorescent properties [10], as agrochemical products [11], [12], [13], [14] such as insecticides or fungicides, claimed as charge generation agents for electrophotographic photoreceptors [15], studied as non linear optical materials [16] and investigated as stain developers for the identification of nucleic acids [17], [18]. Barbituric acid derivatives are also useful in chemical transformations, having been used as organic oxidisers [19], used in the synthesis of unsymmetrical disulfides [20] and as intermediates to oxadeazaflavines [21], [22], [23], [24] which are used as biomimetic models of the 5-deazaflavine coenzyme [25], [26], [27]. The barbituric acid moiety has also been used effectively in some isoindolinone and azo pigments [28] such as Pigment Yellow 139, 150 and 185 which are used to colour inks, paints and plastics.

Environmental pressures and legislation have gradually seen the decline in use of lead chrome (Pigment Yellow 34) and cadmium pigments (Pigment Yellow 37 and 44) in the plastics and automotive paint industry [29]. This has left a gap in the palette of stylists because the excellent properties and shades obtained from these pigments cannot be achieved by the use of the organic pigments alone, presently on the market. The results of this are easy to see. There are few yellow cars on the roads, and those we do see tend to be dull shades. This is because iron oxide or bismuth vanadate pigments have usually been used and these are inherently dull yellows.

Yellow organic pigments such as the isoindolinones Pigment Yellow 109, 110 and 173 are used to a certain extent in the automotive paint industry but on the whole, these suffer from poor durability becoming more intense (darkening) on weathering, especially in full shade [30]. The pigments that are used for industrial vehicles are the diarylide Pigment Yellow 183 and the benzimidazoles Pigment Yellow 151 and 154. These pigments do have the long term durability for use on industrial vehicles which are generally repainted after two years. The copper azomethines Pigment Yellow 117 and 129 are of true automotive quality but are used only in small amounts because they are an unattractive dull greenish yellow and the anthrapyrimidine Pigment Yellow 108 has only a small market because it is weak, very expensive and not completely durable. One method of achieving bright yellow cars is to put down a base coat of bismuth vanadate followed by a benzimidazole or isoindolinone coat and finally a clear coat containing an overdose of UV absorbers. This procedure is extremely expensive and is only used for some sports cars, where auto OEMs tend to work on the safe side having experienced hefty claims in the past for paint failure, in particular due to intercoat adhesion problems.

Yellow is one of the most popular colours in plastics with demand historically met by cadmiums, lead chromes and more recently condensed azos and benzimidazolones. In the plastics industry, the cadmium pigments are no longer used for economic reasons where pigment property requirements such as lightfastness and durability are less strict, eg. polyvinylchloride and low density polyethylene, or for ecological reasons where high demands such as high processing temperatures are made on the pigments in the plastic.

Lead chromate pigment usage dropped significantly in the nineties [31] on health and safety grounds. In the EC, lead chromate pigments are not permitted for use in colouring plastics for consumer goods or coating toys [29]. The concerns over safety of certain pigment types are not only restricted to those based on heavy metals. The future of diarylide pigments in plastics has been called into question because the higher processing temperatures required for certain plastics has led to the thermal decomposition of the pigment leading to unacceptable levels of dichlorobenzidine in the finished product. Unfortunately, as processing temperatures increase, the number of useful, yellow organic pigments used in this application decrease.

Hence, there is a need for high quality, organic yellow pigments with good lightfastness, migration resistance, weather resistance, hiding power and high thermal stability to fill these gaps. Another useful feature for these types of pigment would be to give the plastic good dimensional stability when it is injection moulded.

It has been established that organic pigments with the best durability are those with the most efficient packing in the solid state. The most important, intermolecular interactions that contribute to this efficient packing, are π–π stacking and hydrogen bonding [32], [33]. The barbituric acid group present in the yellow and orange isoindolinone and azo pigments provides hydrogen bonding within these pigment systems giving them good durability but this still falls short of the standard required for automotive quality. The aromatic component of these pigments and hence the π–π stacking is relatively low compared to pigments with excellent durability and it was reasoned that if this could be increased, it would lead to an increase in durability.