Heat stabilizers synergetic mixtures

Polyepoxide plasticizers comprise two basic types. The first type is the epoxidized soybean oils (with a maximum iodine number of 6 and an oxirane oxygen minimum of 6%), which have a good affinity to vinylic polymers [45]. Compared to plasticizers of the second type, the alkylepoxy stearates (butyl, octyl), they are less volatile and are less effective for low-temperature plasticization. Polyepoxide plasticizers have less affinity to PVC than primary plasticizers. They are efficient as heat and light stabilizers and for this reason they are used in synergetic mixtures to stabilize PVC. 

None of the antistatic agents combine all the properties needed, i.e., antistatic efficiency, heat stability, miscibility with the basic polymer, nontoxicity, hygienic acceptability. The easiest practical answer to this problem is to use mixed antistatic agents showing a synergetic effect. It concerns either the mere mixtures of two or more antistatic agents or the so-called internal mixtures, which means that the same molecule contains many effective atoms. 

Synergetic mixtures of metallic soaps (mainly Ba, Ca, Cd, and Zn) have found wide application as heat stabilizers [16,23,24,26,30,32,34-37,45,46, 48-53]. Synergism, in this case, is the action of two heat stabilizers achieving a higher stability than the best offered by only one of the two (see Section 10.2.6). 

Synergetic mixtures presenting no health hazard are worthy of mention. They are zinc-based compounds mixed mainly with calcium soaps. Since zinc-containing stabilizers function only at very low concentration in PVC (0.1-0.4% by weight), it is necessary to ensure a good dispersion of the stabilizer mixture in the polymer matrix to obtain the best stabilizing effect. Too high a concentration of zinc stabilizer decreases the heat stability of PVC appreciably [55]. To avoid this, epoxide compounds should be used as HCl acceptors in conjunction with the stabilizer mixture. 

Fig. 9.7. Relation between heating time at 180°C in the air and the quantity of HCl evolved from PVC stabilized by a synergetic mixture of barium and cadmium stearates [65]. Curve 1 sheet obtained by homogenization in a twin-roll mixer curve 2 homogenized powder in a turbomixer.

The work herein has shown the suitability of P-DSC as an accelerated oxidation stability test method for biodiesel. Different biodiesel blends (B7) were compared in terms of their oxidative stability by P-DSC using dynamic (linear heating up to oxidation) and isothermal modes (maintaining constant temperature at a fixed value up to oxidation). The OOT and OIT parameters were determined for each studied blend and the positive influence of chain-breaking type additives (such as amine and/or phenolic compounds) has been shown. BHT showed better results (high OOT and OIT values) than TBHQ, but EAP, which is a mixture of 1 amine and 2 phenolic compounds, was by far the best AO. Synergetic effects could explain these very good results. 

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