Connecting The Dots Project
Understanding the functions of pigments in plastics
To improve plastic recycling rates, it is essential to analyze the factors influencing the recyclability of different plastics. This article focuses on examining the role of pigments in plastics.
Pigments are insoluble particles, whether organic or inorganic, blended with polymers to provide specific colors to plastics. While their primary purpose is coloring plastics, pigments also contribute unique properties such as heat resistance, UV protection, flexibility, and more. Pigments are generally classified into two main categories: organic and inorganic pigments.
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Organic Pigments:
Organic pigments consist of carbon atoms forming strong, stable chemical bonds and can be derived from plants, animals, or synthetics. They present challenges in terms of mixing and dispersion, often leading to the clumping of pigment particles, resulting in visual inconsistencies in the final products. Organic pigments are chosen for their ability to provide intense and vibrant colors, although they tend to be relatively expensive.
Inorganic Pigments:
Inorganic pigments, primarily composed of minerals, are typically more cost-effective. However, they tend to be more toxic than organic pigments due to the presence of lead salts. Unlike organic pigments, inorganic varieties, including metal oxides, sulfides, and carbon black, disperse more easily within the resin. Titanium dioxide, a widely used inorganic pigment, offers attributes like odorlessness and absorbency. It is employed to enhance whiteness and opacity, serving as a bleaching and opacifying agent in porcelain enamels, providing brightness, hardness, and acid resistance. Inorganic pigments are preferred when high opacity is required.
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Major performance properties of pigments in plastics
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Weatherability or aging
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Light fastness
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Flexibility, warping, or nucleation
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Transparency
The characteristics and potency of pigments can vary from one type to another. Let's compare organic and inorganic pigments in the context of the color orange.
Example 1: Organic orange
As you can see on right. There are a few family of orange pigments within organic pigments. They all have unique properties and usages.
Example 2: Inorganic orange
This is inorganic orange pigment. Some colors have more families than the others.
Other Unique Pigments
Carbon Black: Carbon black stands out as one of the most versatile and widely used black pigments for thermoplastic and anti-static applications, including automotive fuel caps and pipes. It also serves as a common filler in plastics, films, adhesives, and paints. Carbon black is known for its excellent color strength, cost-effectiveness, and UV-resistant properties, attributed to its small particle size and high oil absorption. Its versatility allows it to be used for color modification or tinting.
Aluminum Pigments: Aluminum pigments find their application in various polymer types to introduce metallic functionality and enhance aesthetic qualities.
Fluorescent Pigments: Fluorescent pigments and light-conducting pigments possess unique characteristics, collecting and re-emitting light, making them valuable in diverse applications.
Article sources:
https://www.britannica.com/science/titanium-dioxide
https://polymer-additives.specialchem.com/selection-guide/pigments-for-plastics
Problem with the recycling of black and pigmented plastic
Carbon black's unique characteristic of absorbing near-infrared light creates challenges in the recycling of black plastics. Sorting sensors cannot detect reflections from these materials, resulting in a significant portion of black plastics ending up in landfills. This issue extends to products with darker pigments in general, which often necessitate processing into black or darker shades or disposal in landfills. Additionally, brightly colored items, such as PET, present recycling difficulties because their vibrant hues can contaminate the recycling process, preventing them from being recycled alongside clear or white PET products.
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