Nickel dithiocarbamates

Many PD antioxidants that contain sulfur are ineffective light stabilisers and some even act as photoprooxidants. However, the nickel dithiocarbamates (NiDRC) and the analogous dithiophosphates... 

Fig. 5. Time-controlled photodegradation of LDPE stabilized with different concentrations of the photosensitize iron dithiocarbamate (FeDRC) and the photostabilizer nickel dithiocarbamate (NiDRC). Numbers on curves are antioxidant concentration in 10- mol/100g(156).

The nickel dithiocarbamates (Vllb) are hydroperoxide decomposers [21]. Unlike the zinc analogues, they are light stable and effective UV screening agents below 340 nm [30]. They react rapidly with hydroperoxides as they are formed in the photo-oxidizing polymer to give sulphur acids which are catalysts for hydroperoxide decomposition [21] and are themselves converted to nickel sulphate (see Scheme 1.5). This chemistry leads to the eventual depletion of the metal thiolate in the polymer and at this point (the end of the induction period), the rate of photooxidation reverts to that of the polymer containing the uncomplexed transition metal ion. 

They are converted to polymer-bound metal carboxylates in photooxidizing polyolefins which act as highly effective photo- and thermal prooxidants [9, 13, 24], while they themselves all behave initially as thermal and photoantioxidants. The zinc and nickel dithiocarbamates (II. M = Zn, Ni, n = 2) by... 

Figure 9.3 Comparison of the controlled degradation of LORE sensitized with an iron dithiocarbamate with the stabilizing behaviour of a nickel dithiocarbamate. (Numbers on curves are additive concentration in mol/100 g x 10". ) (From S Al-Malaika, A. Marogi and G. Scott, J. App. Polym. Sci., 31,685 (1986), with permission.)...

On the other hand, dithiocarbamate nickel and zinc complexes are very effective light stabilizers for polyolefins. Using a combination of iron(pro-oxidant) and nickel or zinc (stabilizers) dithiocarbamates, it is possible to control the rate of photo-oxidative degradation very accurately [1931]. For long induction periods a combination of iron and nickel dithiocarbamates is recommended, whereas for shorter times it is more usual to combine iron and zinc dithiocarbamates [1932]. The combination of different metal ions decreases the induction period and increases the rate of photo-oxidation. At higher concentrations of stabilizing (Ni, Zn) dithiocarbamates, much longer lifetimes can be achieved. 

One hectare of land requires up to 150 kg of mulch film. Powder residues and buried fragments represent 0.003% (without taking account of dispersal due to mechanical action). Buried strips which have not decomposed under the effect of light amount to 7% of 0.003% (0.0002% or 2 ppm). These strips (buried and not decomposed) come to the surface the first time the soil is turned over and are destroyed by the same mechanism as the exposed film. After a year the powder particles break down into H2O and CO2. Photoinitiators used in these films contain iron and nickel and are probably iron and nickel dithiocarbamates (cf. section 5.9). 

These are another category of materials to the filler and pigment screens mentioned previously. These materials are generally organic and include derivitives of thiophen, benzotriazole and transition metal dithiocarbamates, e.g., Ferro 101, nickel dithiocarbamate. These substances are effective at concentrations of 0.1 % or less. UV curable acrylate coatings and inks have been used in the manufacture of printed polymer containers. 

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