Investigations of the Thermal Stability and Degradation Mechanisms of Several Polymer-Additive and Copolymer-Additive Systems (1993)

An ever increasing demand for the synthetic polymers and the fire hazards associated with these polymers requires some knowledge of the processes occurring in a fire situation and how these hazards could be at least minimised. Chapter 1 deals with some of the processes which could occur during controlled thermal decomposition of polymer systems. There is a brief account of the type of fire retardants, degradation in their presence and how fire retardants can work. Since traditional halogenated fire retardants have been proved health hazard, there are brief notes on the non-halogenated fire retardants which are found to be effective. This chapter also includes the aim of the present work. Chapter 2 summarises the apparatus and experimental techniques employed in this research. The first section describes the thermal analysis techniques while the second summarises analytical methods used to identify the degradation products. Characterisation and thermal degradations of low density polyethylene (LDPE), poly(ethyl acrylate) (PEA) and ethylene ethyl acrylate (EEA) copolymer are discussed in Chapter 3. EEA copolymer is found to be more stable than PEA but less stable than LDPE. Thermal decomposition of EEA copolymer is found to be initiated at weak points and the degradation products to be mainly the sum of degradation products from LDPE and PEA. The systems are found to be more stable in an inert atmosphere than in air because oxidation results in the formation of groups such as hydroperoxides which lead to in the formation of ketones and peracids at lower temperatures. Chapter 4 gives a brief account of the history of silicones, their industrial applications and physical properties of silicon and its compounds. This chapter also describes the work carried out on thermal degradation of silicone polymers by other research workers. Finally, thermal degradation of polydimethylsiloxane (PDMS) with different end groups is described. The main degradation products are found to be cyclic siloxane oligomers, with cyclic hexamethylsiloxane being the major product. The degradation products are formed by Si-O bond rupture but the mechanism of degradation depends on the chain end groups. It is also found that PDMS is more stable in air than in an inert atmosphere. In Chapter 5, the thermal degradation of poly olefins in the presence of coated CaCO3 is described. EEA copolymer is also considered with Mg(OH)2, A1(OH)3, MgO, TiO2 and different sized (coated and uncoated) CaCO3. It is found that calcium carbonate does not stabilise LDPE. CaCO3 interacts with polyolefins if polar groups are introduced by copolymerisation. The mechanism for the formation of some degradation products changes. It is observed that stabilisation of blends of EEA copolymer with metal hydroxides depends on the endothermic decomposition of metal hydroxides while Mg(OH)2 also interacts with the ester groups and stabilises the copolymer, hence changes the mechanism of the degradation products. Most fillers prevent the formation of acids by forming ionic salts with the acid groups introduced during the decomposition of ethyl acrylate. TiO2 stabilises the copolymer initially but the degradation products are similar to those form the pure copolymer although some degradation routes are more favoured than the others. Stabilisation of EEA copolymer increases in air in the presence of CaCO3 since the filler possibly reacts with the acidic groups produced due to the oxidative attack and stabilises the system. The thermal degradation of blends of polyolefins with high molecular weight vinyl end grouped PDMS is discussed in Chapter 6. All blends form high levels of insoluble rubbery residues due to crosslinking introduced by radical reactions between the components. It is observed that quantity of residue and stabilisation of polyolefins with polar groups increases with increasing percentage content of PDMS. It has been found that initial stabilisation of PDMS with LDPE is only shown in dynamic nitrogen atmosphere while PDMS is stabilised under all conditions with other systems. Decomposition of PDMS in such systems occurs after the decomposition of the organic polymer. The mechanism of degradation changes and new siloxane products are formed resulting mainly from the radical reactions between the components or as a result of radical centres introduced into PDMS chains. The influence of various inorganic fillers and some other additives on the thermal degradation of PDMS is investigated in Chapter 7. It was observed that fillers and additives interact with PDMS (with either end group) and result in great stabilisation of the polymer.