Melt stabilization, polymers

Wachsen, O., Platkowski, K., and Reichert, K-H. (1997). Thermal degradation of poty-L-lactide-studies on kinetics, modelling and melt stabilization, Polym. Degrad. Stab., 57,87-94. 

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. 

As with the polysulphones, the deactivated aromatic nature of the polymer leads to a high degree of oxidative stability, with an indicated UL Temperature Index in excess of 250°C for PEEKK. The only other melt-processable polymers in the same league are poly(phenylene sulphides) and certain liquid crystal polyesters (see Chapter 25). 

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). 

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)...

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. 

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. 

Henman, T.J. Melt stabilization of polypropylene. In Developments in Polymer Stabilisation Scott, G., Ed. Applied Science London, 1979 Vol. 1, 39-99. 

Eastman Chemical Company Product Literature [www.Easlman.com]. Minnick, L. A. and Seymour, R. W., Poly(l,4-cyclohexylenedimethylene terephdialate) with improved melt stability, US Patent 5 428 086, 1995. Minnick, L. A., Polyester molding composition, US Patent 4 778820, 1987. Auerbach, A. B. and Sell, J. W., Evaluation of poly(l,4-cyclohexylenedi-methylene terephdialate) blends for improved processability, Polym. Eng. ScL, 30, 1041 (1990). 

Examples of widely used secondary antioxidants are phosphites, phosphonites, and sultides (Fig. 11.7). Usually, secondary antioxidants are used in combination with primary antioxidants to benetit from a synergistic effect. The main action of phosphites and phosphonites is the oxidation to the corresponding phosphates by reacting with hydroperoxides. These P compounds are mainly used as melt stabilizers during processing. Sulfur compounds act as well as hydroperoxide decomposers via sulfur oxide and sulfenic acid formation. Sulfur compounds are preferably used in combination with phenolic antioxidants to improve the long-term thermal stability of polymers at temperature ranges between 100 and 150 °C. 

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