MELT STABILITY

Whilst conventional polycarbonate based on bis-phenol A is essentially linear, branched polymers have recently been introduced. These materials have flow properties and a melt stability that makes them particularly suitable for large (20 litre) water and milk containers. Branched polymers have also been used in the manufacture of twin-walled sheet for the building industry. 

Transformation products of stabilizers formed during melt processing may exert either or both anti- and/ or pro-oxidant effects. For example, in the case of BHT, peroxydienones, PxD (reactions 9b, b") lead to pro-oxidant effects, due to the presence of the labile peroxide bonds, whereas quinonoid oxidation products, BQ, SQ, and G- (reaction 9 b, c, d) are antioxidants and are more effective than BHT as melt stabilizers for PP [29], The quinones are effective CB—A antioxidants and those which are stable in their oxidized and reduced forms (e.g., galvinoxyl, G-, and its reduced form, hydrogalvi-noxyl, HG) may deactivate both alkyl (CB—A mecha-... 

Table 2 Melt Stabilizing Efficiency of Antioxidants in PP (processed in an internal mixer at 190°C). Melt flow index (MFI) Measured at 230 C and 2.16 Kg...

Another approach to safer stabilization is to use a biological antioxidant such as vitamin E (a-tocopherol is the active form of vitamin E, AO-9, Table la). It is essentially a hindered phenol which acts as an effective chain breaking donor antioxidant, donating a hydrogen to ROO to yield a very stable tocopheroxyl radical, a-Tocopherol is a very effective melt stabilizer in polyolefins that offers high protection to the polymer at very low concentration [41], (Table 2). 

Figure 1. Effect of dicumyl peroxide (DCP) and processing conditions (excess, OM, and restricted, CM, oxygen amount) on melt stability of PP.

For a given level of fuel at a given thickness with the same halogen content, most halogenated compounds show more or less the same flame retardancy. The key differences among these FR additives are effects on flow, melt stability, mechanical properties and long-term ageing of the FR-PBT blend. Different end-use requirements may call for the addition of different FR additives. 

Certain organophosphorus compounds can be used to melt-stabilize PET. Stabilizers such as 3,5-di-t-butyl-4-hydroxybenzyl diethyl phosphate (Irganox 1222) and triphenylphosphate lead to a reduction in the concentration of terminal carboxyl groups of PET, thus giving improved hydrolytic stability. 

Minnick, L. A. and Seymour, R. W., Poly(l,4-cyclohexylenedimethylene terephthalate) with improved melt stability, US Patent 5 428 086, 1995. 

DMPPO—polystyrene blends, because of the inherent flame resistance of the DMPPO component (oxygen index ca 29.5), can be made flame retardant without the use of halogenated additives that tend to lower impact strength and melt stability in other polymers. Approximately one-half of total Noryl sales volume is in flame-retarded grades, ie, V0 or VI in a 1.6-mm section (UL-94). 

Fig. 2 Effect of extrusion temperature on melt stability of PP processed with 300 ppm tocopherol and on its level of retention in the polymer (Reproduced with kind permission of Polym Degrad Stab, 1999, 64 145)...

Due to their peroxidolytic activity, phosphites are normally used in combination with hindered phenols in order to deactivate hydroperoxides formed as a consequence of the antioxidant function of hindered phenols, see reaction 1. The enhanced melt stability of tocopherol-containing PP extruded in the presence of a phosphite is clearly illustrated in Fig. 5a with an optimum melt stabilising performance at a 1 2 w/w ratio in favour of the phosphite. 

Degradation of a hydrocarbon polymer starts at or before the processing stage and in many aspects this history of the sample is the least studied parameter of the degradation. The combination of high temperature, low oxygen concentration, and shear stress as it occurs, e.g., in extruders is very difficult to simulate in the laboratory. For this reason, the development of melt stabilizers is still very empirical, the problem being considerably less studied than that in the case of other stabilizers. 

The effective but volatile 2,6-di-ferf.butyl-4-methyIphenol (11) was replaced for most applications by various mononuclear (12), binuclear (13), trinuclear (14) and tet-ranuclear phenols (15, 16). Recently, synthetic DL-a-tocopherol (17) was introduced for effective melt stabilization of PO. This antioxidant may be listed among the stabilizers generally recognised as safe (GRAS). 

Alkylated benzo[b]furan-2(3H)-ones (19) is another non-traditional stabilizer introduced very recently (Pospfsil and Nespurek, 1997 Irganox, 1997) for high-tempera-ture melt stabilization of PO. It provides an outstanding color maintenance and is used in amounts of 0.01-0.02 % in stabilizer blends consisting of hindered phenols (such as 12, 14, 16) and aromatic phosphites (total amount of the stabilizer blend is approximately 0.1 %) (Irganox, 1997). Most probably, stabilizer 19 is able to scavenge both radicals POO and P, the latter by recombination with a carbon-centered radical generated from 19. 

Alkyl and aryl phosphite esters are also effective melt stabilizers. They are often used in combination with hindered phenols to give highly efficient melt stabilizing systems and to reduce discoloration of the polymer because of the oxidation products of the phenols present. Phosphites (particularly those derived from aliphatic alcohols and unhindered phenols) are, however, generally susceptible to hydrolysis. Consequently, moisture-sensitive phosphites affect adversely the handling characteristics (i.e., flow properties) of the additive package and are a source of other problems corrosion of metal surfaces, formation of dark colored spots, and gel formation. In practice, hydrolysis-resistant phosphites based on sterically hindered phenols are used, e.g., AOs 17 and 18, Table 1. 

Al-Malaika, S. Goodwin, C. Issenhuth, S. Burdick, D. The antioxidant role of a-tocopherol in polymers II- melt stabilizing effect in polypropylene. Polym. Deg. Stab. 1999, 64 (1), 141-156. 

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