Optical Brighteners Recycle Old Colors
Brent D. Johnson
As plastics age, they become oxidized and yellowed through photoexposure. In consumer products, this can be a problem because containers and packaging that have lost their luster can be a disincentive to purchasing an item. This is a major concern with recycled plastics that Eastman Chemical Co. of Kingsport, Tenn., has formulated a series of optical brighteners called Eastobrite that can improve the whiteness of recycled plastics. They are variations on a fluorescent powder called thiophenediyl benzoxazole, which can be added to polyester, polypropylene or nylon.
When you see white, explains product manager David Darden, it may have a red, green or blue cast to it. The human eye perceives yellow light as bad (something that is worn or aged) and blue light as good. That's why laundry detergents contain blue dye crystals, he said.
When a brightener is put into polymers or fibers, it absorbs UV light in the 340- to 400-nm range and re-emits the light in the blue end of the visible spectrum from 400 to 500 nm. It also is slightly fluorescent, which makes fiber and plastic appear brighter and whiter.
To ensure its optical properties, the powder is measured using a tristimulus reflection colorimeter. The company puts the substance into solution using a strong solvent and uses a spectrophotometer to compare it with a standard. The method of quality control is based on the Cielab color scale, using the L*, a* and b* color values indicating lightness, green/red and blue/yellow, respectively. When brighteners are added to polymers, they impart lower b* values that add a deep blue cast to the material.
A Hunter Lab Ultrascan XE spectrophotometer is used to obtain the yellowness and whiteness index. The instrument is configured to accommodate powder samples, which may have nonuniform characteristics such as light trapping between particles and sensitivity to ambient light. It also has a UV control option that compensates for sample fluorescence.
When qualified, the brighteners are dry blended with a master batch of polymer for extrusion compounding and for forming pellets from 25 to 450 ppm. The treated plastic can be used in everything from lampshades to mini blinds.
Some plastics are incompatible with the brighteners, however, because of the polarity or insolubility of the host polymer. When this happens, it's like mixing oil and water. Flexible polyvinylchloride, for example, does not accept Eastobrite because the material migrates to the surface.
A polymer must be matched to an appropriate brightener, too, Darden said. An optical brightener in a Coca-Cola bottle, could make the contents look like motor oil, yet it can make fruit juices look more appealing. Theoretically, brighteners could help keep a beverage from spoiling by converting UV light to visible.
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