Photo-Graftable UV Absorber Gets the Yellow Out

Sanduvor PR-25, a uv absorber based on new photo-reactive chemistry, was aimed primarily at coatings when it was first launched a couple of years ago. Since then it has made headway in clear plastics, including flexible PVC, polycarbonate, PET, and other engineering thermoplastics.

Made by Clariant Corp., Charlotte, N.C., Sanduvor PR-25 is a non-staining, highly absorptive, photo-graftable stabilizer based on benzylidene malonate ester chemistry. It boasts much lower color development than traditional hydroxy-substituted benzophenones and benzotriazoles. It is said to have three times higher absorptivity than these uv absorbers and to also outperform hydroxyphenyltriazines. The latter represent another relatively new uv-absorber chemistry that is now available from a number of suppliers, including Ciba Specialty Chemicals, Tarrytown, N.Y., and Cytec Industries, West Patterson, N.J.

Keeps the yellow out

Clariant's product-development technical manager for additives, Joseph Webster, says PR-25 has a maximum uv absorptivity at least twice as high as that of hydroxy-substituted triazines and three times as great as traditional benzotriazoles and benzophenones.

Clariant initially introduced photoreactive chemistry in the form of Sanduvor PR31, a low-molecular-weight, methylated, monomeric HALS. Sanduvor PR25 has the same backbone chemistry as Sanduvor PR-31, minus the HALS (piperidine) functionality. Like PR-31, PR-25 remains mobile within the polymer until it becomes permanently grafted into place under the influence of light. PR25 is a white crystalline powder with a melting point of 131-134 F.

The four accompanying graphs show the performance of PR-25 in retarding color development compared with traditional uv absorbers or the newer triazines. Data on transparent, compression molded flexible PVC shows PR-25 to outperform Ciba's Tinuvin P benzotriazole, traditionally a workhorse for flexible PVC, after 3000 hr of accelerated weathering.

Similarly, data on injection molded polycarbonate show less color development with PR-25 after 700 hr of accelerated weathering than with either Ciba's Tinuvin 234 benzotriazole (a workhorse for polycarbonate) or Cyasorb HPT 1164 triazine from Cytec.

In molded PET, PR-25 also produces less yellowing before weathering than does Tinuvin 234 (commonly used in PET). The unweathered PR-25 sample is virtually indistinguishable from the control sample containing no uv stabilizer.

More interesting is color development in PET after 600 hr of accelerated weathering where PR-25 still outperforms Tinuvin 234 benzotriazole.

Sanduvor PR-25 uv absorber is priced competitively with the benzotriazoles and newer triazines. It lists at $15.50/lb as compared with $14-28/lb for hydroxy-substituted benzotriazoles and $26-50/lb for the phenyltriazines.