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Need a stabilizer for the long haul? Here's a '15-year' comparison.

Most discussions of polyolefin stabilization concern protection against uv light and/or elevated temperatures. However, even polyolefin products that are stored in the dark at ambient temperature can discolor in various ways over extended periods of time. Such discoloration is attributed to the presence of phenolic antioxidants and to certain types of hindered-amine light stabilizers (HALS).

Conventional studies of initial processing stabilization indicate only an additive's tendency to discolor under the relatively extreme conditions of melt processing. Unfortunately, few data are available to help select appropriate stabilizers or predict their long-term effects on storage discoloration. Long-term testing is cumbersome and results are very difficult to reproduce, short of using a time machine, says Joseph Webster, product manager and principal investigator for additives and chemicals at Clariant Corp., Charlotte, N.C. In what is believed to be the first published study of its kind, his firm evaluated different antioxidants and HALS in LLDPE that was stored at ambient temperature in the dark for 15 years. Significant differences in performance were observed for different primary and secondary antioxidants.

Webster is quick to admit that 15 years is considerably longer than would be considered practical for color stabilization. However, it gives a clue to chemical interactions that are taking place and may possibly be significant at shorter time periods. "It prompts you to ask some questions," he says. For example, Webster notes that some degree of discoloration was observed with an aliphatic phosphite (distearyl pentaerythritol diphosphite). According to the conventional wisdom, that's not supposed to happen, Webster says.

WHAT CAUSES DISCOLORATION?

Phenolic antioxidants are known to generate colored byproducts while doing their job of inhibiting free radicals that would attack the host polymer. Storage discoloration caused by gas staining (also known as gas fading or gas yellowing) involves direct oxidation of phenolic antioxidants by ambient gases such as oxides of nitrogen that result from combustion processes (e.g., gas heaters and propane-powered lift trucks). Such an interaction can occur during storage of either pellets or finished products.

Webster explains that in a total systems approach, one would tailor the type and ratio of primary and secondary antioxidants in the additive package to take into account the temperature and residence time of processing, as well as long-term exposure. For example, in LLDPE rotomolding there is a minimum of two heat histories (pelletizing and molding), plus a mechanical grinding step, all involving high temperatures and long residence times. Polyolefin fiber, film, and sheet also are processed at high temperatures and require substantial residence times.

Recognizing the need for both initial and long-term data, and knowing that it is rare in the industry to monitor samples for an extended period of time, Clariant decided several years ago to dedicate a storage facility to study long-term effects.

DIFFERENCES AFTER 15 YEARS

The company's first results focus on the extent to which commonly used primary and secondary antioxidants in LLDPE perform in long-term storage at room temperature in the dark. Explains Webster, "The aim was to show both initial yellowing and what may happen 15 years later in terms of the stability of a molded LLDPE part."

Pellets of a 0.918-g/cc LLDPE were formulated with three primary phenolic antioxidants: Irganox 1076, 1010, and 3114 from Ciba Specialty Chemicals, Tarrytown, N.Y. Irganox 1076 and 1010 are industry "workhorse" antioxidants, for which generic equivalents are available from many suppliers. They are also the dominant stabilizers for LLDPE. When the study started 15 years ago, Irganox 3114 was designated Goodrite 3114 and was supplied by BFGoodrich Co.

As secondary antioxidants, Clariant used two aromatic phosphites - Irgafos 168 from Ciba Specialty Chemicals and Polygard TNPP from Uniroyal Chemical, Middlebury, Conn. - as well as one aliphatic phosphite, Weston 618, from GE Specialty Chemicals (then Borg-Warner Chemicals), Parkersburg, W.Va. In addition, Clariant tested its own aromatic phosphonite, Sandostab P-EPQ.

The comparison chart at the top of p. 23 shows both initial and long-term results with primary antioxidants alone. The one beneath it shows results with secondary antioxidants alone. After the most severe processing conditions (10 extruder passes, equivalent to a total of 10-12 min residence time at 374 F), the greatest storage stability was observed with Sandostab P-EPQ, followed by Irgafos 168 and then Polygard TNPP. The chart on this page shows initial and long-term results with Irganox 1076 and secondary antioxidants. Not shown here are Clariant's data for Irganox 1010 with secondary stabilizers, which produced even greater discoloration. Also not shown are the results for Weston 618, which is not commonly used in LLDPE. It exhibited some discoloration, though less than with the aromatic phosphites. According to Webster, the data show that blends of Sandostab P-EPQ with all primary antioxidants provided the lowest initial discoloration and the best long-term storage stability.

Clariant is focusing on expanding use of its Sandostab P-EPQ beyond polyolefins and polystyrene to include higher-temperature engineering resins such as ABS, polycarbonate, polyetherimide, and PPS. Webster notes that P-EPQ is not only synergistic with all phenolic and thioester antioxidants, but has also been found to be synergistic with newer stabilizer chemistries such as lactones and Vitamin E.
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Title Annotation:plastics additives
Author:Sherman, Lilli Manolis
Publication:Plastics Technology
Geographic Code:1USA
Date:Mar 1, 1998
Words:850
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