pigment technology: searching for a ‘universal red for plastics’

Pigment technology: searching for a'universal red for plastics'

Pigments are the building blocks of colour in the plastics world and it is through their many combinations that we get the shades we see around us. Of those building blocks, red ones are of particular importance, lan Christensen, Application Manager, Business Line Plastics, of Ciba Specialty Chemicals' Textile Effects Segment discusses the requirements for a universal red pigment for plastics that demonstrates all-round performance and suitability for a wide range of applications. Following the recent development of a non- warping version, he proposes that Pigment Red 254 comes closest to being such a'universal red pigment'.

While buyers and users of plastic articles could be interested in the product's over- all appearance or colour, they are unlike- ly to be concerned about how the colour was achieved. Participants in the produc- tion chain know that getting the right colour in the end product is not a simple matter. The task is to choose the right colorants and disperse them in the polymer from which the article is made. Dispersion of colorants, particularly pigments, is a complex subject and beyond the scope of this article. However, even the choice of colorants to be used is a complicated matter and one that requires careful consideration of many aspects. Obviously the chosen combination of pigments must give the right colour and opacity. However, the colorist must also consider polymer type, processing conditions (particularly temperature), ease of dispersion of the pigment, safety aspects during handling and any effect of the pigment on the polymer. The performance requirements of the end application, such as the length of time the article must last

before fading and whether it will be used outdoors, must also be considered. Satisfying food contact requirements may also be a consideration, as well as the product's ultimate disposal route. Finally, there is the issue of the costs of the pigment, formulation and the whole produc- tion operation. With many commercially available pigments to choose from, it is usually straightforward to find a set of pigments that provides an appropriate technical solution. However, if this philosophy is applied without constraint, a master- batch house or compounder may end up using hundreds of different pigments. The problem of raw material prolifera- tion is significant and has led to a need for pigments to be not only technically effective in one or two respects, but be suitable for a wide range of applications. The development of more versatile pigments that will satisfy the majority of application requirements now makes it possible to reduce the total number of pigments used.

There are a number of considerations when looking for the 'universal red pigment for the plastics industry'. For any pigment intended for a wide variety of uses, satisfactory performance in heat and migration resistance is now essen- tial. It must also be strong, bright, high- ly saturated, and preferably opaque and versatile in formulation. The best all- around pigments will also demonstrate reasonable light and weather fastness, stability and non-reactivity. In addition, they should be unaffected by additives, suitable for many polymers (polyolefins, PVC and polystyrene, at least) and have minimal impact on polymer behaviour and processing, as well as being safe, easy-to-use and cost-effective. In the search for a suitable 'universal red', the product classes (or pigment chemistry types) with the broadest application versatility are the quinacridones and - with the periodic addition of improved prod- ucts having both lower cost and enhanced performance - diketo-pyrrolo-pyrroles (DPP). These materials offer good colour

Plastics Additives & Compounding March 2002

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and technical properties over a wide range of criteria. In addition, they can meet the requirements of a large number of applications and provide a favourable balance of colouring properties. Pigment Red 254 was introduced to the market in the late 1980s and was the first pigment from the new class of diketo-pyrrolo-pyrrole (DPP) pigments. This pigment chemistry satisfies nearly all the requirements specified, including:

• A good mid-red hue, which makes it suitable for bright red applications.

• High saturation (Chroma) and good colour strength, providing formulat- ing power.

• The option of opaque, semi-opaque and transparent versions.

• Good heat and migration resistance. • The durability to make it suitable for

demanding outdoor applications. • Approvals for food contact applica-

tions in Europe, North America and Japan (listed in the French Positive List and by the FDA; 21 CFR 178.3297, no limit by type of polymer)

• Not based on hea W metals. • No sensitivity observed to common

polymer additives • High cost-effectiveness

Product and process development has now resulted in the introduction of new variations of this pigment. For example, Ciba ® Cromophtal ® Red 2030 offers good colouring performance and application versatility, particularly in polyolefins, PVC and polystyrene, while Ciba ® Cromophtal ® DPP Red BOC demonstrates good weather resistance, making it suitable for demanding outdoor applications. Ciba ® Irgazin ® DPP Red BTR exhibits good strength and transparency making it ideal, not only for transparent applications but also for use in combination

with effect pigments such as metallics and pearlescents. The new versions of this pigment have significantly increased its range of suitable applications. However, one area of continuing interest in plastics colouration is control over the influ- ence of pigments on polymer properties. Through the development of new techniques, in some circumstances it has become possible to modify the way in which pigments interact with the polymer matrix. This is important in a number of areas of the plastics industry, particularly in the injection moulding of high-density polyethylene

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(HDPE). Organic pigments tend to affect the shrinkage behaviour of HDPE and in severe cases this can lead to distortion and warping of moulded parts.

Shrinkage and warpage of HDPE Shrinkage of injection-moulded HDPE is a normal phenomenon, well known to plastic processors. It occurs partly due to cooling of the polymer, but also because in the crystalline structure of the frozen polymer where the polymer chains pack more tightly than they do


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Table I : Compar ison of various forms of P i g m e n t Red 254

Strength: the pigment percentage required to achieve a specific depth of colour (1/3 ISD in HDPE).The lowest number is the strongest pigment.

Opacity: the difference in colour of a I mm thick plaque viewed over black and white backgrounds. Lowest number is the most opaque.

Heat resistance: the temperature at which the colour just starts to change after 5 minutes exposure, °C in HDPE.

Weather resistance: the visual assessment of a reduction in colour intensity where 5 means no change and I means substan- tial change.

Warping: tendency to cause warping in HDPE mouldings

Strength Opacity Heat resistance Weather resistance Warping

Cromophtal DPP Red BOC 0.25 21 290 4-5 Low

Cromophtal Red 2030 0.18 18 300 4 High

Cromophtal Red 2028 0.20 18 300 3-4 None

Irgazin DPP Red BTR 0.13 31 280 3-4 Low

in an amorphous state, which means that they occupy a smaller volume. If the shrinkage is uniform and the shape is simple, there may be no further difficul- ty. However, in most artides this shrinkage will be heterogeneous - occurring more strongly in some directions than others. This is due both to orientation of the polymer in the direction of flow and to differences in crystallinity induced by variations in the rates of cooling in different parts of the article. This differential

shrinkage generates mechanical stresses within the part, which can cause it to twist out of its original shape and it becomes distorted or warped. Distortion is influenced by many factors. These include polymer characteris- tics, geometry and gating of the mould, shape of the part, moulding and cooling conditions and the additives present. The mutual interaction between these different factors is complex and difficult to predict.

Within this mix, the rate of crystallization is a key parameter. If crystallization is accelerated, the polymer melt is likely to freeze with the polymer chains still strongly aligned with their flow path. This cre- ates excessive shrinkage in the direction of flow, which finally translates into deformation of the article. The extent of the deformation is clearly increased if the item has little inherent rigidity (thin walls or no internal reinforcement, for example). In this respect, part and mould design play an important role. Most organic pigments have some effect on the shrinkage of HDPE processed by injection moulding. Organic pigments generally consist of microscopic crystals and it is believed that the crystalline, but relatively non-polar surfaces of the pigment crystals initiate alignment of the polymer chains and serve as nucleation sites for the crystallization process. Frequently the presence of an organic pigment causes a major increase in nucleation and crystallization rates, lead- ing to significant differences in the polymer structure (morphology) of a coloured article compared with an uncoloured one. As a result, the shrinkage of a pigmented article can be


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significantly greater than that of uncoloured material. This has major implications for producers of caps and closures where dimensional stability is important, as well as for producers of bins, boxes, crates and other large mouldings where shrinkage and distortion can cause problems with stacking, nesting, twist- ing and even cracking in the most severe cases. New developments have enabled the compatibility of Pigment Red 254 with HDPE to be improved and the risk of warping to be reduced. This further extension of the Pigment Red 254 chemistry resulted in the launch of Ciba ® Cromophtal ® Red 2028. This new version of the pigment has been designed to have a minimal effect on the nucleation behaviour of polymers, which means that the coloured polymer behaves in a similar way to uncoloured polymer. This means that the new pigment is a powerful,

non-warping, red pigment, which is ideally suited for injection-moulding applications and all dimensionally sen- sitive applications.. In pigment technology product performance is not just a consequence of the basic chemistry of the coloured molecule. Particle size distributions, crystal structures and surface chemistry also play vital roles. These other attrib- utes also create the scope for product performance to be optimized in different ways. While all four forms of Pigment Red 254 display high heat resistance and easy dispersability, Table 1 shows some fea- tures of its main commercial forms. The four versions of this pigment that have been developed for plastics make Pigment 254 suitable for a wide variety of applications and so make it the lead- ing candidate for a 'universal red for plastics'. The high weathering form is used in outdoor applications such as toys, crates and automotive components.

The transparent version is used in marking films, toys and in combination with effect pigments. The non- warping version is used for boxes, caps, crates and trays, while the opaque, gen- eral-purpose version has a broad range of applications. Contact: Ciba Specialty Chernica/s Tel: +4I 6I 63 62348 Fax: +41 6I 63 65483 Email: [email protected]

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pigment technology: searching for a ‘universal red for plastics’

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