Pigmentation of polypropylene

Most inorganic pigments show low to moderate color strength combined with good to excellent heat resistance, lightfastness and weather resistance [2]. They are applied in concentrations up to 1% in PP and keep 

Titanium dioxide white pigment based on the rutile modification is mainly used for PP due to its high refractive index of 2.8. ZnS and ZnS/ BaS04-based lithopone pigments are also used due to their low hardness and abrasivity, e.g. in glass reinforced PP types. 

Channel and furnace type carbon blacks, distinguished by their manufacturing process, are of major importance for the coloration of PP. The status of the various carbon blacks for food applications [3] differ according to type and purity. 

Lead chromate yellow and molybdate orange pigments 

These brilliant, inorganic yellow and orange pigments are widely used for PP. Specially treated t) es show adequate heat resistance for PP processing also at higher temperatures. Both pigment types are under legislative pressure in several countries, due to their lead and chromium VI content [4]. 

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In general the selection of pigments for polypropylene follows the same considerations as for polyethylene. Because of the higher processing temperatures and the lesser resistance to oxidation, selection does, however, require rather more care. 

Certain metal salts effectively reduce the photoactivity of titanium dioxide pigments. Combination of these salts with an appropriate antioxidant and/or ultraviolet stabilizer provided highly efficient stabilization of polypropylene. The deactivation/ stabilization performance of the metal salts is adequately explained on the basis of their decomposition of hydrogen peroxide at the pigment surface and by annihilation of positive holes in the pigment crystal lattice. 

P.O.72 is used for melt spin dyeing of polypropylene, in which 1/3 SD (1% TiOz) affords 0.2% pigment. The lightfastness of PP colorations with 0.3%... 

P.Y.177 which was introduced to the market a few years ago, is not listed anymore as a commercial product. It was a special-purpose pigment for polypropylene and polyamide spin dyeing. As a colorant for these media, P.Y.177 has the added advantage of enhancing the stability of the fibers. 

Apart from paints, P.R.224 is also used in polyacrylonitrile spin dyeing. Application in the spin dyeing of polypropylene is compromised by the fact that medium to high pigment concentrations accelerate the degradative action of light on HALS stabilizers (Sec. 3.4.1.4). 

The mixture of P.O.43 and P.R.194, which is a mixed phase comprising both isomers, is also used as a pigment. Its main market is in spin dyeing, especially of polypropylene. Almost as fast as P.O.43, its color is referred to as scarlet. The product is thermally stable up to 300°C. 

This view of polypropylene, while partially deserved in its early history, has not been true since the early 1980s because of improved polymerization conditions, stabilizers, and pigments. It was recognized that both the chemical inertness that prevents dyeing and the low melting point can be advantageous in many products. 

Fabrics used in the automotive industry must meet exceptionally high requirements and be subjected to nonstandard tests that simulate the high-temperature and humidity conditions of light exposure [7]. Dyed fibers cannot meet these accelerated exposure specifications, which are met by specially stabilized, pigmented polypropylene fibers. The outdoor use of polypropylene fibers is expected to grow because of improved stabilizers. It has permitted grass-substitute products to be manufactured in a variety of forms. Many of them resemble carpets, which, until 1980, were used only indoors. 

Because polypropylene contains no polar groups, it contains no dye sites capable of reacting permanently with dye molecules. Since monomers containing polar groups react with the catalysts used to make polypropylene, it has thus far proven difficult to incorporate dye sites during polymerization. Therefore, coloration of polypropylene fibers has been accomplished either by (1) modification of polypropylene as part of the fiber-manufacturing process to render the resultant fiber dyeable or by (2) the addition of pigments. 

Many pigments, particularly organic yellow, orange, and red, have negative effects on the photostability of polypropylene. 

Structure of Polypropylene Fibres Coloured with Organic Pigments... 

Phthalocyanine and quinacridone belong to organic pigments, which are commonly used for the coloration of polypropylene fibres. The chemical formulae of pigments are presented in Figure 1. 

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