Pigments for Plastics: Complete Technical Guide

What are pigments?

• Pigments that are organic in nature are hard to disperse and tend to form agglomerates (clumps of pigment particles). These agglomerates can cause spots and specks in the final product.
• On the other hand, inorganic pigments (like metal oxides, sulfides, and carbon blacks) get more easily dispersed in the resin. Amongst the inorganic pigments titanium dioxide is the most widely used pigment in the plastics industry.

How are pigments classified?

• Organic pigments
• Inorganic Pigments
• Carbon blacks
• White pigments
• Special effect pigments
• Aluminum pigments, and
• Other pigments

Typical performance properties

1. Weatherability/Aging – Exposure to sunlight and some artificial lights can have adverse effects on the useful life of plastic products. So polymers that are frequently used outdoors need UV protection. Hence, they also require weather-resistant pigments.
   
   To assess weathering resistance in practice, you must use outdoor exposure tests in the climatic region(s) concerned. This is not always feasible. The widely used alternative is accelerated testing: CYCLE WOM 119.
   
   We must consider the light stabilizer system together with:
   
   
   • the pigment formulation and
   • the specified fastness criteria for the final product.


2. Light Fastness – It is a measure of the color fastness of light over time. This property measures the color change of plastics when used in indoor applications. Pigment selection for plastics used in indoor applications depends on:
   
   
   • polymer type
   • concentration of the pigment
   • presence of titanium dioxide (which typically accelerates fading)
   • required light fastness
   • service conditions
   
   
   If a pigment has good Light Fastness, it does not always mean that it has good weather-fastness.
   
   Pigment performance can also be influenced by:
   
   
   • surface of the article
   • processing heat history, and 
   •  stabilization package
   
   Generally, inorganic pigments exhibit superior light fastness than organic pigments.


3. Warping/Nucleation – The two factors that determine the final properties of a plastic article are:
   
   
   • the degree of crystallinity and
   • the speed of crystallization
   
   Organic pigments influence these parameters during the cooling phase of plastic processing. Particularly in HDPE injection molding.
   
   This can result in:
   
   
   • Reduced mechanical stability
   • Impact on dimensional stability
     • Warping
     • Shrinkage
   
   Typical applications where this effect becomes important are containers, crates, and caps & closures. Pigments can be divided into three groups, depending on their influence on the shrinkage of HDPE injection molding systems. This distortion tendency is described as:
   
   

   Non-warping	No significant influence, either in lab tests or in practice
   Low-warping	Slight influence, as detected in lab tests, but has been successfully used in practice
   Warping	Significant influence, both in lab tests and in practice. Not recommended for this application or only in combination with specific, warpage-reducing additives



4. Transparency – Usually, transparency is obtained by reducing pigment particle size as possible. This is achieved by surrounding the particles as soon as they are formed with a coating, which prevents the growth of crystals. The most common products used for this coating are rosin or rosin derivatives. This is particularly useful for printing ink pigments that have high transparency. It has the added advantage that such pigments are more easily dispersed.
   
   Iron oxide pigments can be opaque or transparent. The transparent variety is an important group of inorganic pigments as they are widely used for metallic finishes. Their high level of transparency gives them an attractive finish. Their weatherability resistance improves the weatherability of pigments with which they can be combined. This is known as a synergistic effect. Transparent iron oxides depend on the particles being unusually small, and having a crystal shape.
   
   
   • Effect of Dispersion on Transparency - The dispersion process can influence transparency. It involves breaking up agglomerates of particles into individual primary particles. However, primary particles are not split up by the dispersion process. All one can do is to make full use of the pigments original particle size. Good dispersion will maximize the transparency of a small particle.
   
   
   • Measurement of transparency - It is assessed by applying the coating over a black and white contrast chart, and measuring the color difference. The greater the color difference, the higher the transparency.

What are the types of organic pigments suitable for plastics?

• Polycyclic 
• Azo (Mono- and Di-) 
• Metal complexes 

• Chemical structures 
• Surface properties 
• Crystallinity 
• Particle size and size distribution 

Yellow Pigments

Orange Pigments

Brown Pigments

Red pigments

Blue pigments

Violet Pigments

Green Pigments

What are the popular inorganic pigments used in plastics?

• Easy dispersing
• Heat stable
• Lightfast
• Weatherable
• Opaque
• Insoluble, avoiding migration tendencies

Yellow Pigments

Orange Pigments

Brown pigments

Red pigments

Blue pigments

Violet Pigments

Green Pigments

Carbon black: How do they influence plastics performance?

• excellent color strength,
• cost-effectiveness, and
• ultraviolet performances.

Influence on Plastics Performances

• Stability against UV radiation
• Mechanical properties
• Electrical conductivity

White pigments: What are the labeling requirements?

• PVC window profiles,
• agricultural films, and
• in applications that desire chalking stability and color retention.

• The substance TiO₂ must be classified as a carcinogen if inhaled Carc. 2, H351 (inhalation) when supplied on its own or in mixtures. The substance or mixture contains 1% or more of TiO₂ particles with an aerodynamic diameter ≤10 μm. In addition, mixtures containing TiO₂ must be labeled with the supplemental label element ‘Hazardous respirable dust may be formed when used. Do not breathe dust’ (EUH212).


• Non-classified solid mixtures must also be labeled with the EUH212 supplemental labeling element. This classification implies only if they contain at least 1% of TiO₂, regardless of their form, or particle size.


• Liquid mixtures containing TiO₂ do not require Carc. 2 classification. However, if they contain at least 1% of TiO₂ particles with an aerodynamic diameter ≤10 μm, then they need to be labeled with the supplemental label element ‘Hazardous respirable droplets may be formed when sprayed. Do not breathe spray or mist’ (EUH211).

Special effect pigments & types: How are they unique?

• Evoke quality, durability, luxury, or tradition
• Impart an emotional dimension upon the product
• Reinforce branding and differentiation
• Provide the material with a sophisticated, fancy, or attractive appearance

Major types of special effect pigments

• Pearlescent pigments - They impart a pearly or metallic sheen to the products they are added to.


• Interference pigments - They produce shifting colors depending on the angle of light and the viewer's perspective. They create an iridescent effect and are often used in plastics.


• Thermochromic pigments - They exhibit reversible color changes at specific transition temperature ranges, which is useful for application such as food and pharmaceutical packaging (indicating storage for example).


• Phosphorescent pigments - They can absorb and store light energy and then emit it slowly in the dark, creating a glow-in-the-dark effect.


• Chromatic pigments - They change color depending on the viewing angle and the angle of incident light. They create a color-shifting effect and are used in various applications.


• Fluorescent pigments - Fluorescent pigments and light-conducting pigments have intrinsic light-collecting properties. They collect daylight along the edge and re-emit it more intensely, producing a brilliant glow-in-the-dark effect. They can be used to give increased visibility of printing and signs and signals.


• Photochromic pigments - They change from colorless to highly-colored when exposed to ultraviolet light (such as sunlight) and revert when they are removed from radiation. These are organic pigments and exhibit excellent coloration in a wide range of polymers. They can be mixed to from the spectrum of colors.

Aluminum pigments: When to choose between multiple attributes?

• They can also impart great functional performances to plastics.
• They can also provide additional value to your products.

Pigment Form, Carrier Options and Surface Treatment

• a liquid plasticizer such as mineral oil,
• a phthalate ester, or
• a solid resin.

Influence of Geometry on Effects

• Median Particle Size - Usually measured in microns. It is an indication of the average cross-sectional length of the aluminum pigment. The particle size value is usually expressed as the D(50%). This represents the size at which 50% of the flakes in the distribution are larger and 50% smaller. Larger particle size grades are brighter, more reflective, & have more sparkle than smaller sizes.


• Aspect Ratio - It is the ratio of cross-sectional thickness (T) into the cross-sectional length (D). Aspect ratios from 100 to over 1000 are typical of most milled aluminum pigments. Aluminum pigment grades with higher aspect ratio values will typically have:
  
  
  • greater surface area and
  • better coverage or hiding power.


• Particle Size Distribution - It is the measure of the difference between the size of the largest and the smallest flakes of any particular grade. The particle size distribution is characteristic of any single grade but can vary greatly from one grade to another.
  
  
  • Broad distribution ranges have greater hiding power and coverage. They do not produce clean colors when used in combination with chromatic pigments.
  • Narrow distribution ranges produce cleaner colors when used with transparent pigments or dyes. But they need higher loading levels to build opacity.

Influence of 3D Shape on Effects

Aluminum Pigment with Cornflake Geometry

Aluminum Pigment with Lenticular Geometry

Aluminum Pigment with Spherical Geometry

Metallic Effects of Aluminum Pigments

• Glitter Effect - It is a bright metallic appearance with coarse grain sparkles. It is very similar to that obtained when using chopped foil pigments in the 50 X 50 micron to 400 X 400 micron range. Often used to enhance the 3D appearance of an object. This is done by using a low enough concentration of metallic pigment to maintain some polymer transparency. The Glitter effect can be made even more appealing by adding color with transparent pigments or dyes.


• High Sparkle Effect - It is the traditional metallic effect obtained by using aluminum pigments in polymers. Characterized by a strong metallic sheen and often a noticeable grain. It uses a combination of metallic pigment concentrations & transparent or opaque chromatic pigments. They produce broad effects characterized by distinct metallic sheen and sparkle


• Metallescent Effect - It is a soft diffuse translucent metallic appearance. It is similar to the effect obtained by using pearlescent pigments. This effect, or Metallescent appearance, is developed only when using the proper aluminum pigment. The pigments are used at very low loading levels in conjunction with transparent colorants.


• Pinpoint Sparkle Effect - It is unique due to the spherical and polished surface of the aluminum pigments used to produce it. The effect is seen as a cascade of shimmering pinpoint reflections of light. This effect can give "depth" to transparent polymers. This is done by producing sparkling reflections that come from below the polymer surface. When used with transparent colorants and viewed under bright light this effect is quite dramatic. As an added benefit, the Pinpoint Sparkle effect can help reduce the appearance of flow lines in injection molded parts.


• Liquid Metal Effect - It is a bright metallic effect. Produces the appearance of pure metal such as brushed aluminum or polished steel. Characterized by a smooth liquid metallic sheen and controlled grain or no grain. The Liquid Metal look can also produce the appearance of anodized aluminum. Aluminum pigments used to produce the liquid metal effect give polymers the appearance of having been made from solid metal.

 

Functional Performances

• Metallic Luster,
• Reflectance,
• Opacity, 
• Surface Chemistry,
• Particle Geometry,
• Average Particle Size, and
• Particle Size Distribution.

Light Reflection

• the natural ability of aluminum metal to reflect light and
• the lamellar shape of the aluminum pigment particle.

Opacity

• the opaque nature of aluminum metal and
• the high surface area of the lamellar pigment particle.

Gas & Vapor Barrier

• the impermeable nature of aluminum metal and
• the lamellar shape typical of aluminum pigments.

Thermal Conductivity

• greater thermal conductivity,
• lower thermal emissivity, and
• different thermal expansion characteristics than most polymers.

What are the processing solutions for pigments used in plastics?

Heat Resistance

• Heat resistance in PVC - EN 12877-3
• Heat resistance in Polyolefins - EN 12877-2 Procedure A
• Heat resistance of inorganic pigments
• Heat resistance of organic pigments

Dispersion

• Color inconsistency, loss in color yield, batch-to-batch shade deviation,
• Negative effects to the physical and mechanical properties of the final product such as loss of electrical properties in wire & cable applications,
• Negative processing effects such as filter blockage, breakage of the fiber or undesired splitting of the blown films,

Compaction

  
Crystals + agglomerates (normal view) (L) & Compaction (normal view) (R)

Find Suitable Pigments for Polymers