Polypropylene bond breaking

These results provide additional confirmation for the mechanism of pyrolysis of simple polyolefins. The absence of monomer in the volatile products, the maxima in the rate curves, and the sharp decrease in the intrinsic viscosity for linear polymethylene (29) and polypropylene (2, 6, 13, 30) all point to an essentially random scission, due to pronounced intermolecular chain transfer, Equation 2. However, deviations appear when a, the fraction of bonds broken, or, what amounts to the same, the number average DP is examined as a function of time. For small a, the former relation should be one of simple proportionality and hnearity in 1/P. Instead, for both polypropylene (6) and polymethylene [see Figure 5, in (29)] curvature appears, indicating a reduction of the scission rate after an initial period of rapid degradation. For polypropylene this has been interpreted as a breaking of weak and normal bonds. Between 250° and 280° C., one weak link per 2.4 X 10 is found (6). At 295° C., the existence of more than two types of bonds would have to be postulated. 

By comparing the results for chlorinated polypropylene with those for polypropylene, it can be concluded that the two materials undergo very different pyrolytic reactions. Typical for polypropylene is the formation of fragments of the polymeric backbone with formation of monomer, dimer, etc., or with cleavage of the backbone in random places and formation of compounds with 3n, 3n-1, and 3n+1 carbon atoms (see Section 6.1). Pyrolysis of the chlorinated compound leads to a significant amount of HCI and also char. Very few chlorinated compounds are identified in the pyrolysate, since the elimination of HCI leaves very few chlorine atoms bound to carbons. Some aromatic hydrocarbons are formed by a mechanism similar to that of poly(vinyl chloride) pyrolysis. The elimination of HCI leads to the formation of double bonds, and the breaking of the carbon backbone leads to cyclization and formation of aromatic compounds. The reactions involved in this process are shown below for the case of formation of 1,3-dimethylbenzene ... 

To synthesize polyethylene a double carbon bond in the starting material ethylene (CH2=CH2) breaks to allow attachment to other ethylene molecules resulting in a high molecular weight material or macromolecule. Other polymers which are formed by a similar process include polystyrene (repeat unit or monomer is styrene), polypropylene (monomer propene), poly (methyl methacrylate) where the monomer is methyl methacryalate, 1,4-polybutadiene (monomer is buta-1,3-diene) and 1,4-polyisoprene (monomer is isoprene) which has the same formula as natural rubber. Detaik of how polymers are prepared and processed are presented in Chapter 3. 

One method of classifying plastics is by their response to heat. Thermoplasts, also known as thermoplastic polymers, soften and liquefy on heating and harden again when cooled. The process is reversible and can be repeated. On heating, the weak secondary bonds between polymer chains are broken, which facilitates relative movement between the chains. If the molten polymer is further heated until the primary covalent bonds also break, degradation of the thermoplast follows. Thermoplastic polymers are linear or exhibit branching with flexible chains and include polyethylene, polystyrene and polypropylene (Figure 4.10). 

Py-GC-MS can also be used to identify the polymer type in a plastic (Learner, 2001). The principle of the technique is based on the use of heat to volatilize and break down macromolecules into smaller components capable of being analysed using GC-MS (Table 5.6). Degradation mechanisms for polymers under pyrolysis are free radical processes initiated by bond dissociation due to the heat. The specific pathway followed by a particular polymer is dependant on the strength of the polymer bonds and the structure of the polymer chain. Pathways may be described as random scission, unzipping and side group elimination. Polyolefins such as polyethylene and polypropylene follow the random scission pathway and break into pieces of the original molecule to form... 

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