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Pigments for Plastics: Complete Technical Guide

Pigments are organic or inorganic particles added to the polymer base to give a specific color or functional benefits the plastic.

Are you searching for the detailed information on pigments for plastics?

Check out here different families of pigments and colorants for the plastics industry (organic, inorganic, carbon black...) along with their key features, performance properties, and processing solutions.

Pigments - Major Families & Processing


Pigments for PlasticsPigments are insoluble organic or inorganic particles added to the polymer base to give a specific color to the plastic. 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 and sulphides, carbon black, etc. get more easily dispersed in the resin. Amongst the inorganic pigments titanium dioxide is the most widely used pigment in the plastics industry.

Let's take a look at the broad categories of pigments along with their major performance properties...


Performance Properties of Pigments for Plastics


Major performance properties of pigments for plastics are decided by their:

#1. Weatherability / Aging


Effect of Sun Light on Plastic Products Exposure to sunlight and some artificial lights can have adverse effects on the useful life of plastic products. Consequently, polymers which are used outdoors frequently require UV protection and weather resistant pigments.

To assess weathering resistance in practice you have to use outdoor exposure tests in the climatic region(s) concerned. This is clearly not always feasible.

The widely used alternative is accelerated testing: CYCLE WOM 119

The light stabilizer system has to be considered together with the pigment formulation and the specified fastness criteria for the final product.

#2. Light Fastness


Light Fastness is a measure of the color fastness of a plastic article when used in indoor applications (UV light exposure without direct Water contact) If a pigment has good Light Fastness, it does not always mean that it has good weather fastness.

Pigment Selection for indoor use depends on:

  • Polymer type
  • Concentration of the pigment
  • Presence of titanium dioxide (which typically accelerates fading)
  • Required Light Fastness
  • Service conditions

Pigment Performance can also be influenced by:

  • Surface of the article
  • Processing heat history
  • Stabilization package

Generally, inorganic pigments exhibit superior Light Fastness than organics. Tables below rank Light Fastness of main families of organic pigments.

#3. Warping / nucleation, and


The degree of crystallinity and the speed of crystallization determine the final properties of a plastic article. Organic pigments are known to 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, and not recommended for this application or only in combination with specific, warpage-reducing additives


#4. Transparency


Transparency of PigmentUsually, 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 are required to have high transparency and it has the added advantage that such pigments are more easily dispersed.

Iron oxide pigments can be opaque or transparent. The transparent variety are an important group of inorganic pigments as they are widely used for metallic finishes, where their high level of transparency gives an attractive finish, and 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 also having a crystal shape.

Effect of Dispersion on Transparency
The dispersion process can influence transparency, as it involves breaking up agglomerates of particles to 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
Transparency is simply 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.


 » Continue reading about pigments for plastics and explore:



Families of Pigments for Plastics


Pigment families are mainly categorized into:


Let's study them detail:

Organic Pigments


Organic pigments refer to a wide range of chemical families and cover a wide spectrum of properties. They are mainly used for applications needing high tinting strength and brilliant shades while inorganic pigments are mainly useful where high opacity is needed.

We can distinguish three main categories of organic pigments:
  • Polycyclic 
  • Azo (Mono- and Di-) 
  • Metal complexes 

For each category, classical and high performances pigments are available. The performances of the pigment will depend on:
  • Chemical structures 
  • Surface properties 
  • Crystallinity 
  • Particle size and size distribution 

Now let's take a look at the organic components driving a specific color:


 Yellow Pigments
Family Coloristic Properties Resistance Properties Main Polymers in which Used
Anthraquinone Medium to high color strength, transparency
  • Good heat, light and migration fastness
  • Good to excellent weather to fastness
PS, PP, LDPE, HDPE, PMMA, PC, PBT, PET
Diazo pigments High color strength, wide range of shade and opacity
  • good heat, light and migration fastness
  • Medium to good weathering
PVC, LDPE,HDE,PP, PS, HDPE
Isoindolinone Medium to high color, strength, greenish, reddish yellow, and orange
  • Excellent heat, light and migration fastness
  • Excellent weathering, particularly at low concentration and TiO2 reduction
PVC, PS, LDPE, HDPE, PP
Mono Azo salts Medium color strength
  • Good heat, light and migration fastness
  • Low weathering
PVC, LDPE, HDPE, PP, PS

 Orange Pigments
Family Coloristic Properties Resistance Properties Main Polymers in which used
Benzimidazolone high saturation and opacity
  • Good heat, light and migration fastness
  • Medium weathering
PVC, PS, LDPE, HDPE, PP, ABS
Diketo pyrrolo pyrrole (DPP) very pure and brilliant shade, high color strength, wide range of shade and opacity
  • Good to excellent heat, light and migration fastness
  • Reds show good to excellent weather resistance
PVC, PS, LDPE, HDPE, PP
Isoindolinone medium to high color, strength, greenish, reddish yellow, and orange
  • Excellent heat, light and migration fastness
  • Excellent weathering, particularly at low concentration and TiO2 reduction
PVC, PS, LDPE, HDPE, PP

 Brown Pigments
Family Coloristic Properties Resistance  Properties Main Polymers in which Used
Diazo pigments high color strength, wide range of shade and opacity
  • Good heat, light and migration fastness
  • Medium to good weathering
PVC

 Red Pigments
Family Coloristic Properties Resistance Properties Main Polymers in which Used
Anthraquinone medium to high color strength, transparency
  • Good heat, light and migration fastness
  • Good to excellent weather fastness
PS, PP, LDPE, HDPE
BONA Lake high color strength, pure shade
  • Low heat and light fastness
  • Good migration
PVC, PS, LDPE, PP
Diazo pigments high color strength, wide range of shade and opacity
  • Good heat, light and migration fastness
  • Medium to good weathering
PVC, PS, LDPE, HDPE, PP
Diketo pyrrolo pyrrole (DPP) very pure and brilliant shade, high color strength, wide range of shade and opacity
  • Good to excellent heat, light and migration fastness
  • Reds show good to excellent weather resistance
PVC, LDPE, HDPE, PP, PS
Naphthol Lake high color strength, pure shade
  • Good heat and migration fastness
  • Low light fastness
PVC, PS, LDPE
Quinacridone medium to high color, strength, bluish red and violet
  • Good heat, light and migration fastness
  • Good to excellent weathering, particularly in TiO2 reduction
PVC, HDPE, PP, PS, LDPE, PA

 Blue Pigments
Family Coloristic Properties Resistance Properties Main Polymers in which Used
Anthraquinone medium to high color strength, transparency
  • Good heat, light and migration fastness
  • Good to excellent weather to fastness
PVC, PS, LDPE, HDPE, PP, PET
Phthalocyanine high color strength
  • Good heat, light and migration fastness
  • Good to excellent weathering
PVC, LDPE, HDPE, PP, ABS, PA, PET

 Violet Pigments
Family Coloristic Properties Resistance Properties Main Polymers in which Used
Dioxazine high color strength
  • Good heat, light and migration fastness
  • Medium weatherability
PVC, PS, LDPE, HDPE, PP
Quinacridone medium to high color strength, bluish red and violet
  • Good heat, light and migration fastness
  • Good to excellent weathering, particularly in TiO2 reduction
PVC, HDPE, PP

 Green Pigments
Family Coloristic Properties Resistance Properties Main Polymers in which Used
Phthalocyanine high color strength
  • Good heat, light and migration fastness
  • Good to excellent weathering
PS, PVC, LDPE, HDPE, PP, ABS

Inorganic Pigments


Inorganic pigments are known to be:

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

Some of the inorganic pigments, particularly those containing ions capable of more than one oxidation state (for example Pb, Hg, Cr, Cu, Fe), darken upon exposure. Thermal degradation generally manifests itself as darkening as well. Inorganic pigments are not shear sensitive and offer a good value in use.

Certain inorganic pigments exhibit high IR-reflectivity for a given visible color. Inorganic pigments are used in applications requiring surfaces to stay cool and to withstand outdoor elements, such as roofing, decking, and automotive exteriors. This allows costs and energy saving, but also increases the life of the substrate by protecting it from damage caused by the heat of the sun.

Yellow pigments
Family Heat Resistance Light-fastness Other Properties
C.I. Pigment Yellow 42
(Iron oxide)
160 to 180°C - applicable in rubber
C.I. Pigment Yellow 34 
(Lead chromates)
140 to 300°C - more heat and light stable when encapsulated
C.I. Pigment Yellow 184
(Bismuth Vanadates)
240 to 300°C excellent rising importance in the market place (no heavy metals and good properties)
excellent weather resistance
C.I. Pigment Yellow 53 
(Nickel antimony)
up to 1000°C - Low tinting strength

 Orange pigments
Family Heat Resistance Weather Resistance
C.I. Pigment Orange 20
(Cadmium Sulfide)
400 to 600°C adequate for many applications
 Brown pigments
Family Heat Resistance Light-fastness Other Properties
C.I. Pigment Brown 6
(Iron oxide)
max 180°C excellent applicable in rubber
C.I. Pigment Brown 29 
(Chrome/Iron oxide)
300°C excellent high opacity
C.I. Pigment Brown 31
(Chrome/Iron oxide)
300°C excellent high opacity
C.I. Pigment Brown 33 
(Chrome/Iron oxide)
300°C excellent high opacity

 Red pigments
Family Heat Resistance Light-fastness Other Properties
C.I. Pigment Red 101
(Iron oxide)
up to 300°C excellent available in opaque through transparent form
C.I. Pigment Red 104 
(Mixed Phase Pigment)
140 to 300°C increased by stabilization with water glass during production stabilization with waterglass during production increases weather resistance
C.I. Pigment Red 29
(Ultramarine pigment)
up to 200°C excellent very bluish red shade

 Blue pigments
Family Heat Resistance Weather Resistance Other Properties
C.I. Pigment Blue 29
(Ultramarine Pigment)
300 to 400°C - acid resistance improved by surface treatement
good light-fastness
C.I. Pigment Blue 28 
(metal oxyde)
up to 1000°C increased by stabilization with water glass during production excellent dispersibility
C.I. Pigment Blue 36
(metal oxyde)
up to 1000°C - excellent dispersibility

 Violet pigments
Family Heat Resistance Light-fastness Other Properties
C.I. Pigment Violet 15
(Ultramarine Pigment)
up to 250 °C excellent acid resistance improved by surface treatement
C.I. Pigment Violet 16 
(Manganese violet)
up to 300 °C good -

Top Green pigments
Family Heat Resistance Light-fastness Other Properties
Pigment Green 17
(Chrome Oxide Green)
1000°C excellent highly abrasive
C.I. Pigment Green 19 
(Cobalt-based mixed metal oxides)
300 to 1200°C excellent -
C.I. Pigment Green 26
(Cobalt-based mixed metal oxides)
300 to 1200°C excellent -
C.I. Pigment Green 50
(Cobalt-based mixed metal oxides)
300 to 1200°C excellent -


Carbon Black


Carbon Black Pigments for PlasticsCarbon black is the most widely used black pigment for thermoplastic applications. Its small particle size and high oil absorption bring excellent color strength, cost-effectiveness, and ultraviolet performances. Thanks to its chemical purity, carbon black is a useful pigment for a variety of materials besides plastics, including elastomers, inks and surface coatings.

Carbon black may be applied to create full-shade black plastics or as a tinting pigment to modify the color of the chromatic pigments in plastics. As a single pigment, the black color that carbon black imparts to the plastic medium is referred to as "jetness".

Carbon Black Properties


Property Carbon Blacks
Light Fastness excellent
Resistance to solvents good
Chemical Stability excellent
Heat Stability excellent


Influence on plastics performances


Carbon blacks' particle size and shape influence performance attributes in plastic applications, such as:

  • stability against UV radiation
  • mechanical properties
  • electrical conductivity

Plastic Performance Fine Particles Large Particles
Viscosity + -
Ease of dispersion - +
Wetting speed - +
Tint strengh + -
Conductivity + -
UV absorption + -
Cost + -


White Pigments


White pigment in plastics is mainly derived from Titanium Dioxide. TiO2 is widely used for its efficiency in scattering visible light, and imparting whiteness, brightness, and high opacity when incorporated into a plastic formulation.

Moreover, the ability of titanium dioxide to absorb UV light energy can provide significant improvement in the weatherability and durability of polymer products. This property has established the use of TiO2 in applications such as PVC window profiles, agricultural films, where chalking stability and color retention are desired.


Special Effect Pigments


The addition of special effect pigments into plastics allows formulators to :

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


Aluminum Pigments


Aluminum pigments are used in many types of polymers to impart both AESTHETIC and FUNCTIONAL value. There are an amazing number of different aluminum pigment grades each offering a distinct visual appearance or functional attribute.

Aluminium Pigments Center 

Aluminum pigments can also impart great functional performances to plastics and provide additional value to your products.

Aluminum pigments in dry powder form are dusty and easily become airborne. Once suspended in air, a cloud of aluminum pigment particles will burn explosively when exposed to any source of ignition. It is also common for dry powdered aluminum pigments to collect in the duct work of ventilation systems causing additional safety problems. In order to minimize the dust and explosion potential of aluminum pigments, they are often provided in a form in which a pigment binder or carrier holds the individual pigment flakes together.

For use in plastics, aluminum pigments are most commonly provided as a plasticizer dampened powder or as a pellet in which the flakes are bound together with a solid resin.



Pigment Form, Carrier Options and Surface Treatment


A typical aluminum pigment flake is more than just a tiny fleck of aluminum metal. Although the core of the pigment flake is aluminum metal, this core is surrounded by an extremely thin oxide layer which is itself surrounded by a layer of milling lubricant.

This double sheath of oxide and lubricant is responsible for the way the aluminum pigment behaves in particular applications. The arrangement of aluminum metal core and surrounding oxide and lubricant layers is illustrated in the following diagram.

Arrangement of aluminum metal core and surrounding oxide and lubricant layers

For use in plastics, the milling solvent needs to be replaced with a medium or carrier that is compatible both with the polymer chemistry and the processing applications the polymer will subsequently undergo. Typically the milling solvent is replaced with either a liquid plasticizer, such as mineral oil or a phthalate ester, or a solid resin.

he bond between the pigment particle and polymer matrix. Both organic and inorganic surface treatments are available. Care must be taken to choose a surface treatment compatible with the matrix polymer if maintaining physical properties is important because of the differences in chemistry among the many types of plastics commonly pigmented with aluminum.



Influence of Geometry on Effects


Among the characteristics that help differentiate one grade of aluminum from the next are:


Median Particle Size

The median particle size is usually measured in microns and is an indication of the average cross sectional length of the aluminum pigment. The particle size value is usually expressed as the D(50%) and represents the size at which 50% of the flakes in the distribution are larger and 50% smaller. Generally speaking larger particle size grades are brighter, more reflective and have more sparkle than those with smaller particle sizes.

Aspect Ratio

The aspect ratio is determined by dividing the cross sectional thickness (T) into the cross sectional length (D). Aspect ratios of 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 therefore better coverage or hiding power. The actual hiding power of an individual aluminum pigment grade is also greatly influenced by its particle size distribution.

Particle Size Distribution

Particle size distribution range is a 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. In general aluminum pigments with broad distribution ranges have greater hiding power and coverage but do not produce clean colors when used in combination with chromatic pigments. On the other hand, aluminum pigment grades with narrow distribution ranges are capable of producing cleaner colors when used with transparent pigments or dyes, but require higher loading levels to build opacity.

Influence of 3D Shape on Effects


The geometry or three dimensional shape of an aluminum pigment will influence the aesthetic effect it is capable of producing. There are three basic geometries:

  • Cornflake
  • Lenticular
  • Spherical

Of the three, cornflake and lenticular are the most common.

Cornflake

As its name implies, pigments with cornflake geometry have a shape similar to that of cornflakes. The peripheral edges are fairly jagged and uneven and the planar surface is uneven and rough. The rough edges and surfaces of pigments with cornflake geometry scatter light as it’s reflected. This produces a diffuse reflectance that reduces specular reflectance and metallic travel. The following photomicrograph is of a typical cornflake pigment grade.

Aluminum Pigment with Cornflake Geometry
Aluminum Pigment with Cornflake Geometry

Lenticular

The second common lamellar geometry is lenticular or sometimes called round flake geometry. Flakes with this shape tend to have smooth rounded peripheral edges and smooth even planar surfaces. When properly oriented, aluminum pigments with this geometry reflect more light at the angle of the incident light with improvements in specular reflectance and metallic travel. The photomicrograph below shows pigments typical of this geometry.

Aluminum Pigment with Lenticular Geometry
Aluminum Pigment with Lenticular Geometry
 
Spherical

The third pigment geometry is spherical. Much less common than either of the two lamellar geometries, pigments with a spherical shape provide a different type of aesthetic appearance. These pigments provide much less opacity due to their lower surface area. Light reflected from the polished surfaces of aluminum pigments with a spherical shape produce a pinpoint sparkle effect. The combination of low opacity and pinpoint sparkle produce a shimmering reflectance giving the appearance of depth to transparent polymers.

Aluminum Pigment with Spherical Geometry
Aluminum Pigment with Spherical Geometry


As an added benefit, when properly formulated, spherical pigments can help reduce the appearance of flow lines in injection molded objects. This is possible because the brightness of the light reflected from a spherical pigment is much less dependent on the viewing angle as compared to a lamellar pigment.


Metallic Effects of Aluminum Pigments


Metallic Aluminum SheetAluminum pigments are used in many types of polymers to impart aesthetic value. There are an amazing number of different aluminum pigment grades, each offering a distinct visual appearance.

The appearances are divided into 5 major effects:


Let's understand these effects in detail:

Glitter Effect Description

The Glitter effect is a bright metallic appearance with coarse grain sparkles. The appearance is very similar to that obtained when using chopped foil pigments in the 50 X 50 micron to 400 X 400 micron range. The Glitter effect is often used to enhance the three dimensional appearance of an object 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 Description

The High Sparkle effect is the traditional metallic effect obtained by using aluminum pigments in polymers. It is characterized by strong metallic sheen and often a noticeable grain. A variety of metallic pigment concentrations used in combination with transparent or opaque chromatic pigments is used to produce a broad spectrum of effects characterized by distinct metallic sheen and sparkle.

Metallescent Effect Description

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

Pinpoint Sparkle Effect Description

The Pinpoint Sparkle effect is unique due to the spherical shape 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 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 Description

The Liquid Metal aesthetic is a bright metallic effect which produces the appearance of pure metal such as brushed aluminum or polished steel. It is 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 gives polymers the appearance of having been made from solid metal.

Functional Performances



Although most commonly used for aesthetic effects, aluminum pigments also add value by virtue of their physical attributes. These attributes can improve the functional properties of polymers in a variety of ways.

The ability of aluminum pigments to add functional value to polymers is due to the nature of the parent metal and the three dimensional characteristics of the pigment particle. These characteristics include: Metallic Luster, Reflectance, Opacity, Surface Chemistry, Particle Geometry, Average Particle Size, and Particle Size Distribution. Some commonly recognized and other less known applications for aluminum pigments based on their functional attributes are:


Light Reflection

Due to the natural ability of aluminum metal to reflect light and the lamellar shape of the aluminum pigment particle, aluminum pigments are often used to impart reflective properties to blown film. Aluminum pigmented blown film is commonly used in agricultural applications to reflect sunlight back onto plants with the effect of improving crop yield.

The metallic luster produced by light reflected from aluminum pigments is also used in applications with polymers to replace bulk metal. In these applications the polymer takes on the appearance of being made from solid metal while exhibiting the structural properties of the polymer itself. The value of this application includes weight reduction and the ability to produce complex shaped parts without having to be machined.

Opacity

Since light is reflected but not transmitted by aluminum pigments, they are often used to impart opacity to polymers. Typical aluminum pigments have a lamellar geometry which maximizes their surface area. Due to the opaque nature of aluminum metal and the high surface area of the lamellar pigment particle, aluminum pigments are capable of building high opacity at low loading levels. Light blocking window shades are often made by laminating a thin opaque aluminum pigmented film between the visible outer layers of the window shade.

The opacity imparted by low loading levels of aluminum pigmented film is also used for trash bags and shopping bags. In these applications a combination of both the reflective and light blocking abilities of aluminum pigments is apparent.

Bags made with aluminum pigmented blown film keep the contents hidden while providing an appearance of strength due to the metallic luster of the aluminum pigment. The combination of opacity and metallic luster are also used in blow molding applications, for consumer product containers. In this application, the container not only keeps the contents hidden, it also provides protection from harmful UV radiation.

Gas & Vapor Barrier

Aluminum pigments can improve polymer resistance to gas and vapor transmission. This is accomplished by the impermeable nature of aluminum metal and the lamellar shape typical of aluminum pigments. Aluminum pigment flakes with a fairly broad particle size distribution range will orient themselves in an overlapping manner in most processing applications. This configuration blocks any direct pathway through the polymer. Gas or vapor molecules must take a convoluted and greatly lengthened pathway in order to penetrate the polymer.

A thin wall container pigmented with lamellar aluminum flakes will offer the gas and vapor protection of a much thicker wall container. It should be noted that in order to provide effective barrier protection the aluminum pigment flake must be bonded to the polymer matrix during processing. This can be accomplished by using aluminum pigments with the proper surface treatment.

Thermal Conductivity

In certain applications it is desirable to produce a polymer object with the thermal properties of metal. By comparison, aluminum has greater thermal conductivity, lower thermal emissivity, and different thermal expansion characteristics than most polymers.

By pigmenting polymers with aluminum pigments having high aspect ratios, the polymer will start to take on the thermal properties of aluminum metal. Polymers loaded to 40% by weight with a high aspect ratio aluminum pigment have been shown to exhibit heat transfer rates of 80% to 95% that of the pure metal. This is important when making polymer based utensils such as warming trays for use in cooking applications.

Light Reflection Opacity Gas & Vapor Barrier Thermal Properties

Pigment Form Specifically Developed for Easy Use in Plastics



Other Pigments in Polymers


In addition to the pigments described in the above sections, there are other pigments that are used in polymers. Find out about these pigments in the section below:

Fluorescent Pigments


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

Photochromic pigments 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.

Thermochromic pigments

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


Processing Solutions for Pigments of Plastics


While processing, you definitely need to keep in mind the following:

#1. Heat Resistance


Heat is necessary when processing polymers. However, it can be detrimental to polymer and colorant properties. It is important to know exactly the conditions which will be employed when processing. This is especially true when considering that many plastic articles which are produced have actually been subjected to heat on more than one occasion. This is called heat history. Some pigments show intolerances to such heat histories, with a dramatic drop in properties after more than one process.

Heat resistance is always defined by both temperature and dwell time during which colorant was exposed to that temperature.

Typical test methods used to rank the heat resistance of pigments in plastics polymers are:

  • 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

#2. Dispersion


Excellent pigment dispersion in the final product is essential to its performance. Defects caused by non-dispersed pigment particles (specs) can result in a severely lowered performance level. With good dispersion of pigments the following problems can be avoided.

  • 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,
The combination of optimized processing conditions, pigment selection, and dispersing agent can help you achieve good dispersion and avoid some of the problems listed above.

The more finely the pigment is dispersed (smaller average particle size), the larger the total surface area of the pigment particles and the greater its color strength.

Pigment Dispersion

In some cases, dispersion of organic pigments is quite difficult to achieve. The use of predispersed pigments is recommended when coloring engineering plastics and for some PVC applications. They offer greater shade consistency and reproducibility. They remain unaffected by agglomeration by high shear high speed mixers during dry-blending, and have higher color strength (at representative comparable concentrations), purity and transparency.

#3. Compaction


Compaction is the re-agglomeration, by mechanical means, of the pigment to form hard particles that are virtually impossible to disperse. This can take place in mixers, feeders, or compounding machinery. Compaction occurs when pigments and polymers are exposed in the solid state, to increasing pressure and friction. Certain pigments can be more prone to this phenomenon than others. The sensitivity of pigments to compaction depends upon the physical properties of the specific pigment. These phenomena can be avoided by handling, storing and generally treating the colorant carefully.

The reagglomeration tendency is illustrated in the table below.

Crystals + agglomerates
Crystals + agglomerates (normal view)
Crystals + agglomerates (normal view)
Compaction
Compaction (normal view)
Compaction (normal view)



Commercially Available Pigments for Plastics




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3 Comments on "Pigments for Plastics: Complete Technical Guide"
Defab M Feb 6, 2020
Hi, Hope you are very well. could you please let me know what kind of pigments are chemical resistance(Like bleach resistance)? many thanks. regards, Leila
Cintia F Sep 12, 2018
great!! thanks a lot for such interesting article.
Jon P Jul 14, 2018
Well done! The colouration of plastics is a surprisingly broad topic and very little impartial, educational material is available on the web in the public domain. I covered this topic on my own website vibron.com.au , trying to make it approachable by using hand drawn illustrations. It's nice to see someone else attempt to introduce people to this topic.

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