Hindered amine light stabilizer additives

The exterior durabiHty of relatively stable coatings can be enhanced by use of additives. Ultraviolet absorbers reduce the absorption of uv by the resins and hence decrease the rate of photodegradation. Eurther improvements can be gained by also adding free-radical trap antioxidants (qv) such as hindered phenols and especially hindered amine light stabilizers (HALS). A discussion of various types of additives is available (113). 

Figure 13.21 shows the resolution of a dozen polymer additives at very high resolution using chloroform as the mobile phase. Tinuvin 622 will elute in pure chloroform whereas Chimassorb 944 and many other hindered amine light stabilizers (HALS) will not. With the addition of 1% triethyl amine to the chloroform, however, virtually all HALS will elute. 

In the present study possible modes of action of a relatively new class of light stabilizers, the Hindered Amine Light Stabilizers (HALS, tetramethylpiperidine derivatives, TMP) will be discussed. Certain members of this group have a light stabilizing effect superior in many fields of application to that of previous additives. They are effective in both thick layers and fibers13 12. 

Set B Based on the results of set A discussed below, a third set. Set B, was prepared (Table IV). Cyasorb 531 (a benzophenone UV light absorber) was found to be the most effective stabilizer in Set A. It was therefore rerun at an even lower add-on in Set B, and since it was the only UV light absorber included in Set A, a second type of UV absorber, Tinuvin 327 (a benzotriazole) was included in Set B. Two new non-polymeric hindered amine light stabilizers (HALS) Tinuvin 770 and Tinuvin 765 were also evaluated in Set B with the hope that they might be more effective than the polymeric HALS (Chimassorb 944LD) included in Set A which was quite detrimental to the heat stability. Combinations of these four additives were also applied in Set B. The three new compounds evaluated have been reported to be effective light stabilizers for polyester and polyamide fibers (Capocci, G., Ciba Geigy Corporation, Ardsley, NY, personal communication, 1986). 

Other additives include materials such as hindered amine light stabilizers (HALS) and UV light absorbers (UVAs) to protect the colorants from destruction due to the environment, as well as items such as defoamers and wetting or levehng agents to help improve lay down and flow out on the media. Some of these additives such as the stabilizers and the HALS and UVAs must be added in small amounts as they can also impede curing. 

Exposure to solar irradiation (sunlight) may alter their chemical integrity and in due course some physical properties such as mechanical strength (e.g. brittleness) or color (e.g. fading) as well. Additives like industrial ultra violet (UV)-absorbers, hindered amine light stabilizers, radical scavengers and antioxidants are incorporated or applied in these products to effectively reduce or delay such radiation induced deteriorations. 

Tinuvin 292, a liquid hindered amine light stabilizer manufactured by CIBA Specialty Chemicals, found to extend significantly the life of various paints, was tested as an additive at 5% w/w. 

As with thermal stabilizers, photostabilizers must satisfy basic chemical and physical requirements (see the section titled ""Antioxidant permanence effects of chemical and physical factors ). In addition, they must be photo-stable, i.e., stable to UV-light, to withstand continuous periods of UV-exposure, without being prematurely destroyed or effectively transformed into sensitizing products. There are essentially three classes of compounds that are categorized as photostabilizers/ photoantioxidants UV-absorbers and pigments, peroxide decomposers including nickel complexes, and sterically hindered amine light stabilizers. 

A number of antioxidants were examined for solubility in the monomer, effect upon the photoinitiator, UV stability, effect upon physical properties of the cured coating and their effectiveness as stabilizers. The addition of hydrogen donors, HD-2 or HD-3, or a hindered amine light stabilizer, HALS-1, improves the stability of a cured epoxy acrylate system with a minimal change in the physical properties of the resin. DSC and oxygen absorption measurements show that the secondary hindered amine, HALS-1, imparts the best stability to the resin. 

Chemical re-stabilization of recycled material against the thermal and light-induced degradation is essential. Addition of 0.1-0.5 wt% of a sterically hindered phenol and a phosphite at a ratio varying from 10 1 to 1 10 is recommended [Pauquet et al., 1994]. For outdoor applications, hindered amine light stabilizers with UV-absorbers of the benzotriazole type are to be used [Herbst et al., 1995,1998]. Examples of blends used for polymer recycling are listed in Table 1.81. For more details see Chapter 16 in this book. 

As alluded to above, a second class of light stabilizers, the hindered amine light stabilizers (HALSs), provide additional stability to polymers and can function even in thin film or in sample surfaces. The chemistry of HALSs is based on the 2,2,6,6-tetramethylpiperidine derivatives (Fig. 5). Unlike UVAs, HALSs have no significant UV absorption and... 

Metal acetylacetonates and thio-organic complexes show a strong interaction during processing as they yield dark colored samples. These materials are not acceptable as UV stabilizers. Hindered amine light stabilizers (HALS)s are commonly used polymer photostabilizers. HALSs are ineffective in imparting photostability to the polymer. Successful photostabilization of PPS can be achieved by using UV absorbers rather than quencher-type additives. 

Other materials may include pigments and fillers, surfactants, mold release/lubricants, and flame retardants. Coatings will have additional components, including UV stabilizers and hindered amine light stabilizers. 

Additives can also have a similar effect decabromo-diphenyl ether flame retardant in polypropylene was shown to enhance oxidation. Furthermore, where hindered-amine light stabilizers HALS were present in the compound, the degradation product HBr reacted with it to form the ammonium salt that has no photostabilizer effectiveness... 

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