Understanding the functions of pigments in plastics

Plastic color is not simply a design choice. Pigments can affect a product’s appearance, resistance to sunlight and heat, opacity, electrical behavior, and, importantly, its ability to be sorted and recycled.

A pigment is an insoluble colored particle added to a polymer. Unlike dyes, pigments do not dissolve into the plastic. They are dispersed through the resin during manufacturing to create color, whiteness, opacity, metallic effects, fluorescence, ultraviolet protection, or other functional properties. How well a pigment is dispersed matters because poorly dispersed pigment can form clumps, streaks, spots, or weak points in the finished plastic.

Why pigments matter

Pigment selection can influence several important plastic properties:

• Color strength: How much pigment is needed to create a desired color.

• Opacity: How well the plastic blocks light or hides the material underneath.

• Transparency: Whether light can pass through the plastic.

• Lightfastness: How well the color resists fading under indoor or artificial light.

• Weatherability: How well the color and plastic resist sunlight, heat, moisture, and outdoor aging.

• Heat stability: Whether the pigment remains stable during high-temperature plastic processing.

• UV protection: Whether the pigment helps absorb or scatter ultraviolet light that can degrade polymers.

• Dispersion: How evenly pigment particles distribute through the plastic.

• Recyclability: Whether the color allows sorting equipment to identify the polymer accurately.

  
  

Organic and inorganic pigments

Pigments used in plastics are often described as organic, inorganic, or carbon-based specialty pigments.

It is inaccurate to say that inorganic pigments are always more toxic than organic pigments or that they generally contain lead salts. Some historical pigments contained lead, chromium, cadmium, or other metals, but pigment safety depends on the specific chemical formulation, concentration, use, and applicable regulations. Many modern inorganic pigments, including titanium dioxide and iron oxides, are widely used in plastics. Manufacturers should evaluate pigment safety under relevant product, worker-safety, and environmental rules.

Organic pigments

Organic pigments are typically carbon-based compounds. They can be synthetic or derived from natural sources, though industrial plastic pigments are often manufactured for consistent color and performance.

Organic pigments are valued for strong, vivid colors, especially in reds, oranges, yellows, blues, and greens. However, some organic pigments can be difficult to disperse evenly into plastic resin. When pigment particles clump together, they can create specks, streaks, inconsistent color, or defects in the final product.

For an orange plastic product, manufacturers may choose among several families of organic orange pigments. The best choice depends on the needed shade, transparency, heat resistance, lightfastness, price, processing conditions, and end use.

Organic pigments are not inherently better or worse for recycling than inorganic pigments. Their effect on recyclability depends on the pigment chemistry, concentration, color, additives, polymer type, and whether recycling equipment can accurately identify and sort the item.

Inorganic pigments

Inorganic pigments are often based on mineral compounds, metal oxides, sulfides, or other inorganic materials. They can provide good heat stability, weather resistance, lightfastness, and opacity.

Titanium dioxide, written as TiO₂, is the most widely used white pigment in plastics. It scatters visible light efficiently, giving plastic whiteness, brightness, and opacity. It can also help reduce ultraviolet damage to some polymers by absorbing or scattering UV energy.

Examples of inorganic pigment types include:

• Titanium dioxide for white and opaque plastics

• Iron oxides for red, yellow, brown, and black tones

• Chromium- and cobalt-based pigments for certain greens and blues

• Complex inorganic colored pigments for durable colors and specialty applications

• Certain mineral-based pigments used for heat-resistant or weather-resistant plastics

For orange, inorganic options may include combinations of red and yellow pigments or specialized orange mineral pigments. The choice depends on desired color, opacity, processing temperature, cost, chemical compatibility, and product-safety requirements.

Carbon black

Carbon black is one of the most common black pigments used in thermoplastics. It is valued for strong color, low cost, ultraviolet resistance, and use in applications such as automotive parts, pipes, films, adhesives, coatings, and some antistatic or conductive formulations.

Its particle size, surface area, and oil absorption can affect color strength, dispersion, conductivity, and UV performance. Carbon black can be used to create a deep black appearance or to darken and modify other colors.

However, conventional carbon black can create a major recycling problem.

Why black plastic is difficult to recycle

Many material-recovery facilities use near-infrared, or NIR, sensors to identify plastic types such as PET, HDPE, PP, and PS. The sorter shines near-infrared light onto an item and analyzes the reflected signal to identify the polymer.

Conventional carbon black strongly absorbs near-infrared light. As a result, the sensor may receive little or no usable reflected signal from a black plastic item. The machine may be unable to determine whether the item is PET, polyethylene, polypropylene, polystyrene, or another polymer.

When sorting equipment cannot identify the plastic type, black items may be mis-sorted, mixed with incompatible plastics, or sent to disposal rather than recycling. The Association of Plastic Recyclers notes that NIR sorting is not reliable for black or very dark items with low reflectance because conventional carbon black absorbs the NIR wavelengths used for polymer identification.

This problem is not limited only to pure black plastic. Very dark blue, brown, purple, green, and other low-reflectance colors can also be difficult for some sorting systems to identify.

Alternatives to conventional carbon black

Manufacturers can use NIR-detectable black pigments or infrared-reflective pigments that look black to the human eye but reflect enough near-infrared light for recycling equipment to identify the underlying polymer.

These alternatives may include complex inorganic colored pigments, manganese ferrites, and other engineered colorants. Some products using NIR-reflective black pigments can be identified by NIR sorting equipment, improving the likelihood that the plastic can enter the correct recycling stream.

However, an NIR-detectable pigment does not guarantee that an item will be recycled. Recycling also depends on:

• Local collection rules

• Item size and shape

• Polymer type

• Labels, adhesives, coatings, and multilayer construction

• Food contamination

• Sorting equipment

• Market demand for recycled material

• Whether the product is accepted by the local recycling program

Brightly colored plastics and recycling

Bright colors can also complicate recycling, particularly for plastics such as PET. Clear PET is highly valuable because it can be recycled into a wide range of products, including new clear packaging, fiber, strapping, and sheet material.

Strongly colored PET may limit the color and quality of recycled output. A bright blue, red, green, or orange PET container may be recyclable in some systems, but it can be less flexible than clear PET because the recycled material may need to be used in darker products. Colored plastic does not automatically belong in the trash. Always follow your local recycling program’s accepted-material list.

The larger issue is that color can reduce the market value and versatility of recycled resin, especially when colors are mixed or when a small amount of strongly colored material contaminates a clear stream.

Design choices that improve recyclability

Manufacturers can improve the recycling potential of plastic packaging and products by:

• Using clear or lightly tinted plastic when possible

• Avoiding conventional carbon black in packaging intended for NIR sorting

• Using NIR-detectable black pigments when a dark appearance is necessary

• Avoiding unnecessary dark colors, heavy pigments, and opaque layers

• Limiting mixed materials, multilayer structures, labels, sleeves, adhesives, and coatings

• Designing packaging around widely recycled polymers and local collection systems

• Testing products for NIR sortability and compatibility with recycling streams

• Using recycled-content resin where feasible

Key takeaway

Pigments provide color and performance, but they can also determine whether a plastic item can be identified and sorted for recycling. Carbon black is particularly important because it can make black plastic difficult for standard near-infrared sorting systems to detect. Clear, lightly colored, and NIR-sortable plastic designs are generally more compatible with existing recycling infrastructure than very dark or heavily pigmented items.

Sources and Resources

• Association of Plastic Recyclers, Near Infrared Sorting of Whole Rigid Packages in the Plastics Recycling Stream 

• Springer, Black Plastic Waste Classification by Laser-Induced Fluorescence 

• SpecialChem, Pigments for Plastics 

• Ti-Pure, Polymers, Light and the Science of Titanium Dioxide 

• Lanxess, Black Pigment for Recyclable Plastic Packaging 

• Shepherd Color, Infrared-Reflective Black Pigments and Plastic Recycling 

Safety and Legal Notice

This information is for educational purposes only and is not legal, technical, environmental, recycling, manufacturing, or safety advice.

Pigment safety, plastic recyclability, and local recycling acceptance vary by material, product design, recycling facility, and location. Always follow product labels, Safety Data Sheets, manufacturer guidance, and local recycling rules.

Do not assume an item is recyclable because it is plastic or has a recycling symbol. Confirm acceptance with your local recycling program.

Connecting The Dots Project does not guarantee that this information is complete, current, or suitable for every situation.