Colouring Thermoplastics

In order to offer the best colouring solution, one must first know the following criteria.

1. The polymer to be coloured.
2. The end application.
3. The processing conditions, temperature, and dwell time.
4. The type of process such as injection or blow moulding, film or sheet extrusion etc.
5. Indoor or outdoor use (if outdoor what country)
6. Lifetime colour expectation e.g., years or just a few months or weeks.
7. Minimum thickness of the end product.
8. Opacity requirements, e.g., fully opaque, translucent, transparent.
9. Food, toys, or medical applications.
10. Countries where the end product could be used.
11. Any extreme conditions of use during the lifetime of the end product e.g., in contact with temperatures in excess of 100c for long periods of time,
12. Chemical resistance requirements e.g., resistance to Acidic or Alkaline conditions or other specific chemicals.
13. Is the potential for warpage/distortion of real concern.

If the customer is able to divulge this information the experienced colour formulator can then select the most appropriate colourants for the application.

The key to any successful colour match is to ensure the most cost-effective formulation has been derived based on the information supplied.

Sometimes the customer is unable to gather all the information, it is then often left to the experienced colourist to come up with a formula that they consider to be a suitable for the application. The minimum information needed for an experienced colourist to offer a formula is for them to at least know the end application and the country or countries where the end product is likely to be used.

There are four main groups of Colourants used to colour thermoplastics, these are.

1. Dyes
2. Inorganic pigments
3. Organic pigments.
4. Special effect pigments
    

The most common differences between Dyes/Inorganic/Organic pigments are:

Dyes                                Inorganics                                Organic

Typically, higher cost           Less expensive           Moderate to high

High colour strength Low colour strength           High colour strength.

Low opacity                  High opacity                  Low to medium opacity

Easy to incorporate    Easy to disperse           Difficult to disperse.
 

Most colourant’s can be identified by their Colour index numbers, though there are a few suppliers who do not always want to divulge this information, especially if they have invested heavily in making new chemistry grades. Some of the special effect pigments are also unclassified.

Dyes

Dyes dissolve in thermoplastics unlike inorganic and organic pigments which remain as small particulates.

They are only compatible with certain polymers e.g., Styrenics, Polycarbonates, Pet, Pbt, Polyamides and some other more obscure engineering polymers. They are not compatible with Polyolefins due to migration/colour bleed. Care must be taken to choose the most stable of Dyes for the application, as the properties of Dyes can vary tremendously, e.g., heat stabilities can be as low as 200c or as high as 360c.The light fastness and weather fastness of most dyes is generally considered to be quite good.

Dyes tend to get used when transparency is a key requirement e.g. car indicators.

Some of the most widely used dyes in the thermoplastic industry are, Solvent Orange 60, Solvent Red 135, Solvent Red 111, Solvent Red 52, Solvent Violet 13, Solvent Violet 59, Solvent blue 35, Solvent blue 104, Solvent Green 3, Solvent Green 28, Solvent Yellow 114, Solvent Yellow 141.

Inorganic Pigments

These types of pigments are mainly derived from metals or metallic salts or other ground minerals. Most have excellent fastness properties and due to their relatively low cost and good compatibility with most thermoplastics, they are widely used.

The most popular inorganic pigments used to colour thermoplastics are,

Ultramarine blues (Colour Index blue 29) Titanium Dioxide (Ci Pigment White 6) Iron oxides red(Pigment red 101) Iron Oxide yellow ( Pigment yellow 42) Iron oxide Black(Pigment black 11) Carbon black( Pigment black7) Chrome oxide Green ( Pigment green 17) Zinc Ferrite( Pigment Yellow 119) Nickel Titanate( (Pigment yellow 53) Chrome Titanate( Pigment brown 24)

In the past both Cadmium and Lead Chrome pigments were widely used within the plastics industry, but because of concerns about their toxicity, they have all but disappeared, due to the EU and other countries banning their use. Cadmiums do still have a presence in some crucial safety applications where no other colourant is capable of performing to the required colour specification.

Organic Pigments

Organic pigments are based on carbon and can originate from plants and animals, but the vast majority originate from complex organic chemistry, formulated from within laboratories. The most widely manufactured chemical types are. Phthalocyanines e.g. Ci Pigment Blue 15:3, Blue15:1 and Green 7. Quinophthalone e.g. Ci pigment yellow 138 Isoindoline e.g. Ci Pigment yellow 139 and 110 Azo types e.g. Ci Pigment yellows 13,17,  83,  168, 191,183, 180, and 151  Pigment red 53:1, 48:2, 57:1 Perylene e.g.  Ci pigment red 149 and 178 Quinacridones e.g.  Ci Pigment red 122, and Violet 19 Dioxazine e.g. Ci Pigment violet 23. There are many other Organic pigments, but the above list probably accounts for 90% of the total sales going into the thermoplastics market.

The fastness properties of the various types of Organic pigments can vary considerably. Some have excellent light and weather fastness, and can resist any fading for many years, whilst others can start to fade within days. Heat stability can be between 180 and 350c depending on the grade selection, tint strength and opacity are also quite variable. There are also restrictions that come into play should food or toy contact be required, so again care must be taken to ensure the correct organic pigment is selected for the end application.

Special Effect Pigments

There are many types of special effects, not all of which are easy to work with. Many can be sensitive to heat or too much shear during processing, which can destroy the effect, so it’s important to follow the processing parameters outlined by the supplier.

Some of the most common special effect finishes are made from. Pearlescent pigments, to give either a very fine silk finish or large sparkly finish dependant on their particle size, Metallic Bronze, Gold and Silver shades, these can also be supplied as either very fine or larger particle sizes, Speckled/Granite effects made from Mica or other materials also supplied in various particle sizes dependant on the effect requires. Wood effect finishes, as well as Marble, and Fiber effects. There are also thermochromic pigments that change colour depending on the temperature and Photochromic effects that will change colour on exposure to UV light.

So, when it comes to colouring Thermoplastics the choices are many and varied. Designers need to understand what will and won’t work and there will be many occasions where additional help from a particular additive to prevent UV, heat or other forms of degradation will also be required within the formulation. With the right information an experienced thermoplastic colour engineer will be able to formulate the most cost-effective solutions for the application, failure to provide the right information could lead to product failure or an over engineered expensive solution.

Author:

Pravin S Mistry - Global CEO of PREA Ltd and International Polymer Consultancy - Plastics, Composites, Rubber, Adhesives, Polyurethane... focuses globally on:

• Mergers & Acquisitions, buying selling companies
• Recruitment- Full time and Interim
• Interim CEO / MD
• Polymer Consultancy

He personally has worked for over 40 years in the industry - UK, USA, Mainland Europe, Asia... in the polymer other manufacturing industries as Divisional Managing Director and CEO for multinational companies. Early career roles include Operations Director, Technical Director, Technical Quality Manager , Chemical Engineer, Laboratory Technician.