SpecialFocus

by Jeffrey Ross, Ph. D.

he most common additives used in manufacturing polymer sheets are colorants. Customers typically have a strong vision of what they want their product to look like, and the first attribute in generating that look is the color. So how do you generate color? The answer is not always simple, because colorant packages behave differently depending on the use conditions. For example, products intended to be used primarily indoors or for which outdoor use and exposure is short have different color stability requirements than products that are intended to be used extensively, or exclusively, outdoors.

How does an end user determine color requirements? Is it simply understanding interior vs. exterior use? This article can help provide a pathway to first understand how to choose between types of colorants and how to make sure the choices are the right ones.

Different types of light and how it is expressed

Light is all around us and it is often something taken for granted. But not all light is alike, particularly when determining color. Since color interpretation is based on the reflection of light, it is important to understand the type of light that is present in the use environment of a product.

Color is a perception, based on how the human eye interprets visible light that is reflected from objects. Visible light is electromagnetic radiation (figure 1) that is generated by a variety of sources that can be detected by the human eye. The order of colors corresponds to the energy of the light in the visible light spectrum, from red (lowest energy and longest wavelength) to violet (highest energy and shortest wavelength).

Figure 1: The electromagnetic spectrum.

Other portions of the electromagnetic spectrum cannot be perceived by eye but can be detected in other ways. For example, infrared (IR) light is low energy radiation, which is perceived by the human body as heat energy. Ultraviolet (UV) light (figure 2) is higher energy than the visible spectrum and is invisible to humans but can be detected using specialized equipment. The UV portion of the electromagnetic spectrum is divided into three segments: UVA, UVB and UVC. UVA and UVB light are the only components of the UV spectrum that can reach the surface of the Earth, as UVC light is blocked by atmospheric ozone. UVA is typically responsible for sun tanning and premature aging, whereas UVB, the higher energy form, is responsible for sunburn and can cause severe health issues. Recently, manmade sources of UVC radiation have been found to be useful in disinfecting surfaces, including plastics, as certain wavelengths can destroy bacteria and viruses.

Figure 2: UV light spectrum.

In early days of photography, light was described by the temperature of the source of the light. Some examples are D65 (daylight, 6500K), CWF (Cool White Fluorescent, 5000K) and Incandescent (Old style light bulbs, ~3000K) (figure 3). K is a Kelvin, the unit of absolute temperature. If an engineer or designer is specifying materials for use in an outdoor environment, it is important to specify that color matching/selection is done using daylight (D65) light.

Figure 3: Color temperature chart.

Similarly, if the usage environment is indoors, the engineer or designer must first determine what kind of light will be used in the space. The most common types of indoor light are incandescent, fluorescent, daylight and LED. However, there are some specialty lighting systems — such as metal halide and sodium vapor — and, in some cases, clever architectural features allow for daylight as the main type of interior light. Once you know the predominant lighting system, material specifications should always reference that type of light for all color matches.

Now that you have defined the product’s usage environment, the next step in specifying color is ensuring that the colors can survive in that environment. The key environmental differences are between interior and exterior applications, and again the type of light plays an important role. For this part of the specification process, you need to delve deeper into the materials science of colorants.

Pigments or dyes?

There are essentially two types of colorants from which additives for plastics are manufactured: pigments and dyes. Pigments are usually made from minerals that are either mined from the earth or manufactured from materials that are mined. One characteristic of pigments is that most of them are extremely stable and do not show changes in color after lengthy exposure to both visible and ultraviolet light. Common examples of light-stable, so-called “weatherable” pigments, are red iron oxide and white titanium dioxide. Iron oxide is frequently used for any colorant that contains a component of red, including pinks, oranges, tans and almost all wood tones. Titanium dioxide is used to make bright whites and is frequently supplemented by white calcium carbonate (which occurs naturally as limestone). For most plastic colorant additives, pigments are finely ground and then compounded with a plastic carrier using an extruder.

Dyes, in contrast to pigments, are almost all synthetic today, although many were originally discovered and harvested from natural resources. An example is indigo, the color used for blue jeans. There are thousands of dyes available today. The biggest advantage of dyes is the intensity of their color when compared to pigments: A little bit goes a long way. Dyes used in conjunction with pigments can help simplify and accelerate color matching. However, many dyes — particularly blues, oranges and reds — do not retain their color over time in outdoor environments. Because of the potential for fading, dyes are most preferred for interior product applications, for products that have very short lifecycles and for which color fading would not be an issue.

Pay attention to components in additives

A second concern for products to be used in exterior applications is the durability of the polymer from which the product is fabricated. Many plastics perform poorly over time in outdoor environments, with the elements — hot, cold, rain, snow and sunlight (which includes both visible and UV light) — all contributing to fading, discoloration and surface crazing. Fortunately, chemical companies have developed highly effective additive packages for most plastics that can help extend their useful life in the outdoors, sometimes for decades. Typical antioxidant/ultraviolet (AUV) packages available for exterior use applications are described as one-, three-, five- or 10-year packages. These additives almost always include a combination of three things:

• An antioxidant to improve the processing by the manufacturer and to enable re-use and recycling;
• A UV light absorber to intercept harmful UV light and dissipate it without harming the polymer; and
• A hindered amine light stabilizer (HALS) which acts as a synergist to the UV light absorber.

Vycom Celtec^® graphic arts foamed PVC sheets contain a range of colorants.

Vycom TimberLine^® Teak HDPE sheets contain red/brown and special effects colorants, as well as a 10-year AUV package.

Engineers and designers specifying materials for exterior use should always insist on using materials based on proven, exterior weatherable pigments. Many plastics manufacturers and suppliers can test exterior weathering using established ASTM test methods, which provides data on exterior weathering. Look for plastics suppliers that test products using these accelerated weathering methods.

Consider circularity

Finally, before developing a new product, engineers and designers should consider what to do with the product at the end of its useful life. The most preferable end-of-life solution is recycling into the same type of product or into a product that extends the life of the component materials even further. In a circular economy, plastics ae recycled again and again, to eliminate waste and continually use these resources. For polyolefin and rigid PVC products, all can be easily recycled by committed manufacturers with specialized knowledge of sorting and reprocessing, especially if the manufacturer used a robust AUV package in the original fabrication process. As the continued challenge of keeping plastics out of landfills is exacerbated by global commerce changes such as the embargo on plastic scrap exports to overseas companies for processing, it’s on all of us in this industry as manufacturers, distributors and end users to work together to ensure these materials do not end up in landfills.

Jeffrey Ross, Ph. D. is the technical director for Vycom – an affiliate of The AZEK Company. For more information, contact Vycom at 801 East Corey Street, Scranton, PA 18505-3523 USA; phone (800) 235-8320, fax (800) 858-9266, info@vycomplastics.com or www.vycomplastics.com.