Light stabilizers selection

The stabilizers chosen for evaluation include different types of heat and light stabilizers selected to represent different mechanisms of action as well as chemical compositions (ArJi). Types of stabilizers evaluated include benzotriazole and benzophenone light stabilizers [ultraviolet (UV) light absorbers], hindered amine light stabilizers (HALS, catalytic radical scavengers), hindered phenol heat stabilizers (antioxidant radical scavengers), and thioester heat stabilizers (antioxidant hydroperoxide decomposers). 

There are about 40 suppliers of light stabilizers worldwide. Some of these companies also produce antioxidants and PVC heat stabilizers. Of these 40 or so suppliers, only Ciba Specialty Chemicals is a significant player in every region of the world with the broadest product line of light stabilizers. Selected global suppliers of light stabilizers are given in Table 4.18. 

A typical CC-measurement procedure (for a pH-sensitive LAPS structure) is depicted in Fig. 6.3. From the raw data material of the CC-mode measurement of all measurement spots under the pH-sensitive layer, a calibration plot can be derived. For example, for the above example, an average pH sensitivity of 54.2 mV/pH with a standard deviation of 0.5 mV/pH between the different measurement spots can be calculated. This initial calibration measurement allows furthermore the determination of different measurement parameters, e.g., the hysterisis, overall drift, stability, selectivity and the influence of external disturbances such as light and temperature. These parameters are important to evaluate the performance of the complete LAPS-based measurement system. 

Both of these approaches are being explored in an effort to take advantage of the striking improvements in light stability achieved without incurring the real, or at least implied, problems of poor heat stability. Additional stabilizers will continue to be screened and optimum formulations will be selected. 

In Part III of this series titled, "A Light Stability Cabinet for Evaluating the Photosensitivity of Pharmaceuticals," authored by Lachman et al. (38) they discussed the history of photostability testing in allied fields, the different sources that had been used along with their benefits and short-comings, and the rationale for the source they selected. The authors also presented some of the basic theory involved in this field, e.g., the relationship between wavelength and energy. 

Tonnesen HH, Moore DE. Photochemical degradation of components in drug formulations. Part 2 Selection of radiation sources in light stability testing. Pharm Tech 1993 5 27-34. 

The most common UV absorbers used commercially are hindered amine light stabilizers, which can also function as heat stabilizers in some plastics. Other UV absorbers include benzotriazoles and benzophenones, salicylate esters, cyanoacrylates, malonates and benzilidenes. Factors such as the nature of the resin, type and level of pigments being used, and the presence of crosslinkers and catalysts govern the selection of UV absorber to be used. 

Selection of a suitable extraction procedure can increase the concentration of phenolic diterpenes relative to the plant material. In addition, undesirable components can be removed prior to adding extracts to foods. For example, chlorophylls present in plant material may reduce the light stability of food during storage. Several extraction techniques have been patented (Nakatani et al., 1984 Aeschbach and Philippossian, 1989) using solvents with different polarity, such as petrol ether, hexane, toluene, acetone, methanol and ethanol. To obtain tailored extracts, the successive use of different solvents has been applied. 

Such characteristics as compatibility with polymer, volatility, chemical stability under conditions of processing and usage become very important during selection of light-stabilizer. 

The first compound chosen by the above criteria was 4-benzoyl-oxy-2,2,6,6-tetramethylpiperidine (76). The second compound, bis-(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (79) was selected and developed jointly by Sankyo Co., Ltd. and Ciba-Geigy Ltd. In addition we selected a third compound, l-[2-(3,5-di-tert-butyl-4-hydroxy-phenylpropionyloxy)ethyl]-4-(3,5-di-tert-butyl-4-hydroxyphenylpropio-nyloxy)-2,2,6,6-tetramethylpiperidine (83), and developed it as a new type of stabilizer having both thermal and light-stabilization activity. These compounds are widely used under the trade names SANOL LS-744 (76), -770 (79), and -2626 (83) (Chart 12). Compound (79) is also sold by Ciba-Geigy Ltd. as Tinuvin 770 under a licensing agreement from Sankyo. 

Another important consideration in selecting a light stabilizer system for polypropylene yarns is the resistance to activity loss after laundering or dry cleaning. In Figure 5 data are presented... 

Hindered amines are generally classified as light stabilizers. The three selected products are derivatives of 1,6-hexanediamine and are effective antioxidants for polyolefins. 

The most important classes of UV stabilizers are UV absorbers (U VAs), quenchers and hindered amine light stabilizers. In this chapter an overview is given about the types of UV stabilizers in use, their mechanisms of action as well as possible synergistic and antagonistic effects. In the last part for a selected number of polymers the effectiveness of different stabilizers is shown. The chemicals structures of all stabilizers mentioned in this chapter are shown in Appendix 17.9. 

The lack of the melt processing activity of HAS means that they do not contribute to the conventional melt processing formulations consisting of hindered phenols and aromatic phosphites. A properly selected combination of processing stabilizers and HAS has, however, a favourable effect on the LTHS and light stability of the polymer during the longterm application [129,135]. 

Vinyl solution resins are usually made via suspension or solution polymerization techniques. Plastisols are defined as suspensions of homopolymers or copolymers in nonaqueous liquids. The liquids, which are normally vinyl plasticizers, are selected so that they do not solvate the polymer to any extent at room temperature. The suspension is maintained by residual emulsifier left on the particle, and the very small particle size of the polymer itself (all pass through a 200-mesh screen). A finite quantity of plasticizer must be present in order to form the plastisol or "paste" no plasticizer, no plastisol. As with all vinyl systems, consideration may have to be given to plasticization, heat and light stabilization, pigmentation, etc. 

In the following part, the most important classes of additives (antioxidants, PVC heat stabilizers, light stabilizers, and flame retardants) are presented, as well as general information on chemical structures and mechanisms, on testing methods, and on use examples in selected polymer... 

Selecting, combining and matching colors is a complicated art that only well-trained individuals are able to perform correctly. There are hundreds of different colorants used in the plastic industry, and there are as many types of colorants as different applications for plastics. Similarly to measurement of optical properties of paper and paperboard, the characterization of a color for plastics is based on the measurement of color (hue), brightness, and opacity. Other important variables to be considered in colorant selection include dispersability in the plastic, migration, toxicity, light stability, and chemical resistance. 

---