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Ogila et al. – eXPRESS Polymer Letters Vol.11, No.10 (2017) 778–798 P ̇ + P ̇→IP P ̇+POO ̇→IP (19) POO ̇ + POO ̇→IP + O2 (20) where IP are inactive products. The above mechanism was first presented for poly- olefins [67], which make up a majority of the poly- mers used in RM (PE and polypropylene). It has also been used to describe the degradation of polyamides [68] and aromatic esters (polyethylene terephthalate and polybutylene terephthalate) [69, 70] amongst others. Anti-oxidants are necessary to enable the pro- cessing of these polymers as well as to ensure their longevity in service. These take the form of hindered phenols and phosphites [36]. Hindered phenols are radical chain terminators that contain a number of labile hydrogen atoms which they readily dissociate to inhibit free radical formation and more rapidly stabilize the peroxyl radicals already formed. Phos- phites on the other hand are preventive antioxidants that act to hinder the initiation of the oxidation process by preventing the decomposition of unstable polymer hydroperoxides. The thermal degradation mechanism for polyvinyl chloride (PVC) is slightly different from the one pre- sented for polyolefins. The process is caused mainly by structural irregularities in the polymer inherited from its polymerization and is made up of two main reactions. During the initial stage (primary reaction), high processing temperatures cause free hydrogen chloride (HCl) to be evolved by eliminating chlorine from the polymer backbone. This is accompanied by discoloration and the formation of conjugated poly- ene sequences within polymer chains. In the second- ary reaction, polyene reacts with oxygen resulting in peroxyde radicals and hydroperoxides [71]. Decom- position of the hydroperoxides then proceeds accord- ing to the scheme presented by Bolland [67]. Due to the difference in reaction mechanisms, a different set of degradation inhibitors is required for the melt pro- cessing of PVC. Heat stabilizers prevent thermo-ox- idative degradation in two ways: (i) Primary stabilizers, with strong Lewis acid char- acter, react with intermediates in the degradation chain (allylic chlorides) and prevent the forma- tion of polyenes longer than four to five double bonds [71]. (ii) Secondary stabilizers scavenge the evolved HCl and prevent its auto-catalytic effects on chain (18) initiation that can start other dehydrochlorination events [72]. Alkyltin stabilizers are an important group of pri- mary stabilizers. Although they can react with HCl to form tin chlorides, their main contribution to de- hydrochlorination is to react with allylic chlorides. For example, alkyltin thioglycolates can exchange the thioglycolate group for Cl atoms during reaction with allylic chlorides [71]. β-Diketones, epoxidized Fatty Acid Esters and some metal carboxylates also find application as primary stabilizers. Metal car- boxylates based on zinc (Zn) and cadmium (Cd) react in a similar manner to alkyltin stabilizers; how- ever, those base on calcium (Ca), potassium (K) and barium (Ba) only scavenge HCl, and are considered secondary stabilizers. Other secondary stabilizers in- clude phosphites and hydrotalcites. Combinations of both primary and secondary stabi- lizers, that possess synergistic effects, have shown to provide the best improvements in high tempera- ture performance. Indeed, when Kovařík et al. [72] investigated the performance of stabilizers based on CaO, zinc stearate (ZnSt), and the Sterically Hin- dered Phenolic Antioxidant Irganox 1010, they ob- served that the polymer remained thermally stable for more than 60 minutes at 180 °C. Unfortunately, the sheer volume of work related to polymer thermo- oxidative degradation cannot allow a full assessment of all its aspects in this review. It is recommended that further information on oxidation and the stabilizing effects of antioxidants be sought from the compre- hensive reviews given in references [62, 71–76]. Predicting the onset of degradation during RM is a topic of great interest for researchers. This is moti- vated by the fact that molders for a long time deter- mined optimum PIAT largely by trial and error; a method that is as expensive as it is inefficient. The rate constants of elementary reaction during thermal oxidative degradation adhere to the zero order Ar- rhenius equation. Models derived from it analyzed and fairly accurately predicted the thermal stability of various polymers including; PVC [77], polylactic acid (PLLA) [78] and polypropylene (PP) [8, 66] in RM conditions. Of particular significance is the work carried out by Cramez et al. [36], who predicted the onset of degra- dation by determining the time required to complete- ly exhaust antioxidant in a PE; also referred to as ox- idation induction time (OIT). Since temperature 786PDF Image | Rotational molding: A review
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