New Way to Prevent Pigment Swirl in Rotomolding Eliminates Variations Caused by Climate and Enhances Finished-Part Color

Home > Teknor Color Company > What's New > What's New

LODI, OHIO, U.S.A., February 5, 2002: An innovation in antistatic technology for dry color used by rotational molders prevents pigment swirl regardless of atmospheric conditions and yields a more intense hue than conventionally modified colorants, it was announced today by Teknor Color Company. One of the first molders to employ the new technology, leisure products manufacturer Swimways Corp., has reported a dramatic improvement in the appearance of parts produced for its line of swimming pool lounges.

Swirling or streaking of colors in part surfaces results from static electricity generated by friction between the powdered resin mixture and the rotating mold. While conventional antistatic agents are hygroscopic substances that draw on atmospheric moisture to dissipate charge, the additives used in Teknor Color's patented H2Stat (Trademark) system possess sufficient inherent conductivity to prevent static buildup in the first place, according to Harry R. Howard, Director of R&D and inventor of the new technique.

"By eliminating variations that result from differences in humidity from day to day or from region to region, our non-hygroscopic formulations enable rotational molders to produce parts that are consistently swirl-free," said Howard. "And for molders in very dry climates, H2Stat technology provides the first truly effective solution to static buildup."

Another important benefit cited by Howard is more intense coloration of the molded part than that generated by conventional colorants. "Our antistatic additives take the place of the usual dispersion aids and actually do a better job of breaking up pigment agglomerations."

H2Stat technology can be used with polyethylene, polypropylene, polycarbonate, or nylon, and organic or inorganic pigments. Teknor Color will offer the technology in custom dry color formulations at prices only slightly above those of conventional colorants, according to Dennis S. Foy, Market Manager.

An early report on the success of the technology came from Swimways Corp., Virginia Beach, VA, U.S.A., which uses dry color incorporating the H2Stat formulation to rotationally mold "pontoons," flotation devices placed on either side of swimming pool lounges and serving an additional function as armrests.

"I have been involved in rotational molding for 25 years and during that time have become very familiar with the problem of swirl," said David Becker, Manager of Engineering for Swimways. "These are the best results I've ever had. I would say we have reduced swirl to a small fraction of past levels."

Becker noted that use of colorant with the H2Stat formulation enabled Swimways to meet the requirements of a nationwide retail merchandiser that had said it would reject pool lounge products showing swirl.

New Antistatic Technology Also Prevents Premature De-Molding and Pigment Specks
The U.S. patent assigned to Teknor Color (No. 6,051,641) relates to the use of any in a range of non-hygroscopic antistatic agents, alone or in combination, to reduce pigment swirling. Whereas conventional antistatic agents are organic substances, those in the new technology are inorganic additives containing metal atoms that provide conductivity. The additive selected for a given application, its physical form, and its loading in the colorant will vary in accordance with the pigments used and other application requirements, Howard said.

In addition to variations caused by differences in humidity, Howard cited two other problems caused by conventional antistatic agents and avoided with the additives in the H2Stat system. "The organic additives sometimes act as mold releases, causing premature separation of the part from the mold. And because they often have low melting points, they may soften and cause agglomeration of the pigment, resulting in specks in the molded part."

The static charge developed in the powder blend of resin and dry color during the early
stages of the rotational molding cycle cannot be dissipated through grounding because plastics are inherent insulators. "If a difference in polarity arises between the mold surface and the resin blend, the finest particles in the blend will be attracted to the mold surface," explained Howard. "Because these fines are rich in pigment, the surface of the part acquires a stronger coloration than its interior, and, since the resin is moving against the rotating mold, swirl develops."

The hygroscopic antistatic agents developed to reduce swirl require relative humidity levels ranging from 15 to 50% to generate sufficient conductivity, according to Dennis Foy. "By eliminating this dependence on atmospheric moisture, H2Stat technology is the most effective solution yet to a problem that is unique to rotational molding."

In a "pigment swirling index" procedure devised by Teknor Color, observers use the naked eye to gauge the relative amount of part surface area that has non-uniform color distribution. A rating of 15% is equivalent to severe swirling. The effect of the new technology is to totally eliminate swirl or prevent it from exceeding the 3 to 5% range, according to Foy. At typical loadings in the range of 0.2 to 0.5 % by weight of the entire rotomolding compound, he added, the non-hygroscopic antistatic agents exhibit no appreciable effect on the impact strength of the molded part.