No abstract

Mechanical recycling of thermoplastics for high value applications is directly associated with restabilization. The state of the art of processing-, long term heat-and light stabilization of recyclates in different polymers (e.g. PP, PPEPDM, HDPE, LDPE, PS, PVC and various blends) with examples from packaging, distribution, automotive and construction industry is shown. Because of predamage and impurities, recyclates degrade faster and differently compared with virgin polymers and, therefore, specially designed stabilizer systems are required according to the previous damage, subsequent application and type of polymer.Recommendations for restabilization include combinations of phenolic antioxidants and phosphites as well as HALS compounds or "Recyclostab" stabilizer systems.

SynopsisMechanisms for the action of hindered amine light sta bilizers are reviewed briefly. On the basis of these considerations, two aspects of the mode of action of these substances were examined more closely.First the interaction of selected tetramethylpiperidine (TMP) derivatives with radicals arising from Norrishtype I cleavage of diisopropyl ketone under oxygen was studied. These species are most probably the isopropyl peroxy and isobutyryl peroxy radicals immediately formed after α-splitting of diisopropyl ketone and subsequent addition of O2 to the initially generated radicals. Product analysis and kinetic studies showed that the in vestigated TMP derivatives exercise a marked controlling influence over the nature of the products formed in the photooxidative process. The results obtained point to an interaction between TMP derivatives and especially the isobutyryl peroxy radical.Secondly, the interaction of hindered amines with hydro peroxides was examined. At room temperature, using dif ferent monofunctional model hydroperoxides, a direct hydroperoxide decomposition by TMP derivatives was not seen. On the other hand, a marked inhibitory effect of certain hindered amines on the formation of hydroperoxi des in the induced photooxidation of hydrocarbons was observed. Additional spectroscopic and analytical evi dence is given for complex formation between TMP deriva tives and tert.-butyl hydroperoxide. From these results, a possible mechanism for the reaction between hindered amines and the oxidizing species was proposed.

Studies of the photodegradation of polyethylene containing 2 wt % methyl vinyl ketone (MVK) and methyl isopropenyl ketone (MIPK) show that the relative quantum yields of Norrish type I and II photolysis are strongly dependent on the structure of the ketone carbonyl. The type 1 process, leading to free radical formation, is particularly efficient in the MIPK copolymer, somewhat less so in the MVK copolymer, and very low in ethylene‐carbon monoxide copolymers where the carbonyl group is in the chain backbone. In the presence of oxygen, this difference shows up strongly in the rate of photooxidation. Backbone carbonyl groups have very low efficiency in initiating photooxidation of solid polymer films, while the side chain ketone groups initiate rapid photooxidation.

SummaryThe interaction of selected tetramethylpiperidine derivatives with radicals arising from the Norrish-type I cleavage of dibenzyl ketone under oxygen was studied. Product analyses and kinetic studies showed that the investigated sterically hindered piperidine derivatives have a pronounced effect on both the nature and distribution of the products of photolysis of dibenzyl ketone in the presence of oxygen. Observations indicated that the phenylperacetoxyl radical is formed as an intermediate during irradiation and that it interacts with the additives used. Possible mechanisms of the reactions studied are discussed.The observation that oxidation of an isolated double bond by the radicals formed in dibenzyl ketone photolysis under oxygen is strongly inhibited in the presence of the studied sterically hindered amines is discussed in the light of the results presented. The findings are considered in relation to the problem of polymer stabilization.Introduction. -Certain sterically hindered amines (HALS = Hindered Amine Light Stabilizer) have a light-stabilizing action on various polymers (e.g., polyolefins, styrol polymers, polyurethanes) which is far superior to that of any of the stabilizing systems so far used [2-91. The mode of action of this technically and scientifically interesting group of substances has already been the subject of many publications [ 10-201. As all these studies have shown, hindered amines and their derivatives do not act as UV. absorbers. Also they only play a secondary role as quenchers of excited carbonyl groups and singlet oxygen [ll-131 [21] [22]. For this reason, attention was turned at a fairly early stage to the radical-scavenging action of hindered piperidines and especially their nitroxyl derivatives [ 101 [ 141. In the main, interest was centred on the reaction of the nitroxyl with alkyl radicals giving rise to hydroxylamine ethers and the cleavage of the latter by peroxyl radicals [ 141 1201 (reaction (1)).