Brightness reversion of mechanical pulps XIV: application of FWAs for high-brightness, high-yield pulps.

Application: Flourescent whitening agents added to a BCTMP or BCTMP-kraft furnish reduce the photoyellowing properties of paper containing high yield pulp.

Modern mechanical pulp manufacturing technologies readily produce high brightness pulps with pulp yields exceeding 85%. The benefits of this fiber resource include excellent bulk, stiffness, and opacity properties. Nonetheless, the photoyellowing properties of mechanical pulps remain one of the primary factors limiting the use of this fiber resource for a variety of applications. Although the basic photochemical pathways contributing to brightness reversion of mechanical pulps have been studied extensively, no commercially viable solutions are currently being used to halt the overall photoyellowing process.

Recent reports indicate that several new additive technologies can retard brightness reversion to varying degrees. The most effective of these technologies use a mixture of agents to retard photoyellowing. We examined the photostabilization properties of one group of additives--dinminostilbene-based fluorescent whitening agents (FWA)--for a hardwood BCTMP furnish, separately and in conjugation with other known photostabilization agents.

Our research included accelerated photoaging studies using a series of fluorescent office lights and optical properties. We measured the photostabilization effects of FWA against TAPPI brightness standards. The effects were dosage dependent and improved when applied with a carrier additive, such as polyethylene glycol. Alternative antioxidants and UV-screens, used separately or together, also retarded the overall photoyellowing process to some extent.

A 0.1%-0.5% charge of the FWA on handsheets of BCTMP retarded photoyellowing by 25%. Polyethyleneglycol improved the photostabilization effects of the FWA, presumably by improving the surface coverage of the FWA onto the fiber (Figure 1). Brightness measurements excluding the fluorescent component of the FWA-treated BCTMP handsheets suggest the fluorescent whitening agent is most effective in retarding the slow phase of brightness reversion. Using an antioxidant in conjunction with a FWA improved the overall photostability of BCTMR

[FIGURE 1 OMITTED]

We tentatively attributed the observed photostabilization of FWA to the ability of these additives to absorb harmful 340-380 nm light, re-emitting it as a blue light ([lambda] = 425-450 nm). These results provide additional evidence that assorted UV-absorbers and anti-oxidants are effective at retarding brightness of high-brightness mechanical pulps.

CONCLUSION

The use of fluorescent whitening agents to hinder brightness reversion of mechanical pulps appears to be a promising technology for high-yield pulps. We found that the effects of FWA can be improved with the addition of a carrier molecule and with other photostabilization technologies. These results and a recent report by Bourgoing and Robert on the photostabilization of TMP pulps with FWA suggest that these additives may provide an effective technology for retarding the photoyellowing of mechanical pulp.

Arthur J. Ragauskas * is professor, Lenong Allison is research scientist, Lucian A. Lucia ia assistant professor of chemistry, and Cang Li is post-doctoral research scientist at the Institute of Paper Science and Technology, 500 10th St. NW, Atlanta, GA 30318, USA. Address correspondence to Raugauskas by email at [e-mail]