Auto-Accelerated Oxidation of Plastics

The thermo-oxidative degradation of polyolefins and many other plastics is usually an auto-accelerated process, that is, the rate of oxidation is low or negligible in the beginning but steadily increases and often reaches a constant rate of oxidation.

Like ordinary chain reactions, the overall oxidation process can be divided into three stages, called initiation, propagation, and termination of degradation. The first stage, the so-called initiation, is the formation of free radicals via C-C and/or C-H bond cleavage which may be induced by heat, UV light, mechanical stress or by trace amounts of initiators such as peroxides and/or hydroperoxides.

R−H → R· + H·

R−R → R· + R·

Alkyl radicals can undergo a number of "chain" reactions. Propagation of decomposition usually starts when (atmospheric) oxygen reacts with the newly formed free chain radicals R·.1 This reaction produces highly reactive peroxy radicals ROO· which then react with labile hydrogen of polymer chains to form more free radicals R· and hydro peroxides ROOH.

R· + ·O-O· → ROO·

ROO· + RH → R· + ROOH

The second reaction involves breaking C-H bonds which requires considerable energy unless the polymer contains reactive hydrogen. Thus, this reaction is often the rate determining step.

The ROOH can decompose via homolytic cleavage to alkoxy and hydroxy radicals which, in turn, also abstract labile hydrogen from polymers which leads to the formation of even more free chain radicals.

ROOH → RO· + ·OH

RO· + RH →  R· + ROH

·OH + RH →  R· + H2O

However, the cleavage of an O-O bond has a rather high activation energy. For example, the dissociation energy of hydroperoxide is in the range of 200 kJ /mol.2 For this reason, a bimolecular decomposition mechanism with lower activation energy is more likely to occur.2

ROOH + RH → RO· + R· + H2O

The autocatalytic decomposition reactions are usually very fast when compared to (heat induced) initiation reactions (C-C bond cleavage). Since the concentration of hydroperoxide is very low at the beginning of an autooxidation process, the rate of decomposition is very low but steadily increases until the hydroperoxide concentration reaches a steady state which means the rate of peroxide decomposition is equal to that of formation. (See graph below)

 

Effect of Peroxides and Antioxidants
on Autooxidation of Plastics

Inhibition and Retardation

It has been postulated that the bimolecular decomposition of hydroperoxides is the rate determining step for thermo-oxidative decomposition.2 However, this is not always true since even trace amounts of impurities such as transition metals can have a large impact on the oxidative stability of plastics. Particularly, Co, Cu, Fe, Mn can greatly accelerate oxidation because they are potent catalysts for hydroperoxide decomposition and, thus, greatly reduce the activation energy.3

ROOH + M+ → RO· + HO- + M2+

ROOH + M2+ → ROO· + H+ + M+

Unsaturated polymers such as polyisoprene and polybutadiene are most susceptible to metal-catalyzed oxidation because double bonds markedly decrease the activation energy of oxidation reactions. Branching and crystallinity also affect the thermo-oxidative stability of a plastic. In general, polymers with high crystallinity and no branching are more stable than amorphous polymer of low density and low crystallinity such as low density polyethylene. The general order of stability is 2-4

HDPE  >  LLDPE  >  LDPE >  iPP  >  PS  >  PBD  >  PI

To prevent or at least to slow down oxidative chain reactions, antioxidants are usually added. Typically a combination of sterically hindered phenols and organic phosphites are added. The phosphites decompose the hydroperxides into harmless inert products and the hindered phenols scavenge the free radicals.

The oxidation reactions stop when either two radicals recombine (dimerization or cross-linking) or when a chain radical abstracts a hydrogen from another polymer chain (disproportionation / hydrogen abstraction). These reactions always occur but can be accelerated by additon of stabilizers. Recombination of two chain radicals results in an increase of the molecular weight and crossilinking density:

R· + R·  →  R−R

2 ROO·  →  ROOR + O2

RO· + RO· →  ROOR

R· + RO· →  ROR

HO· + ROO· →  ROH  +  O2

These reaction cause enbrittlement and cracking of the plastic. Termination by chain scission, on the other hand, decrease the molecular weight and thus, softens the plastic.

Rn· + Rm·  →  Rn-2−CH=CH2 + Rm

2 RCOO· →  RC=O  + ROH  +  O2

Both chain scission and chain hardening has a negative impact on the mechanical properties. Which of these termination reactions dominates depends on the type of plastic and on the reaction conditions. For example, polyolefins with short alkyl side groups like polypropylene and polybutylene, and unsaturated polymers like natural rubber (polyisoprene) undergo predominantly chain scission, whereas polyethylene, and rubbers with somewhat less active double bonds like polybutadiene and polychloropene become brittle due to crosslinking.

References
  1. Oxygen in its ground state (tripled state) is a diradical ·O-O· which explains its high reactivity.
  2. J.D. Peterson, S. Vyazovkin, C.A. Wight, Macromol. Chem. Phys., 202, 775-784 (2001)
  3. (2) N. Grassie, G. Scott, Polymer Degradation and Stabilisation, Cambridge University Press, Cambridge 1988
  4. E. Chiellini, A. Corti, S.D. Antone, R. Baciu, Polymer Degradation and Stability, 91, 11, 2739 - 2747 (2006)

  • Summary

    Thermo-Oxidative Degradation

    of many polymers is an auto-accelerated process. Oxidation is slow in the beginning but accelerates with increasing concentration of peroxy radicals and often levels off when the peroxide concentration reaches a maximum.

  • Oxidative degradation is usually initiated when polymer chains form radicals, either by hydrogen abstration or by homolytic cleavage of a carbon-carbon bonds.

  • The cleavage of C-C and C-H bonds requires considerable energes and, thus, is often the rate determining step of the auto-catalyzed oxidation of plastics.

  • Thermal-oxidative degradation leads to chain scission or cross linking resulting in the loss of mechanical properties.

  • The thermo-oxidative stability of polymers depends on the C-C and C-H bond energies. The stronger the bonds, the slower the oxidation (bond cleavage and hydrogen abstraction).

  • Highly branched and unsaturated polymers such as polyisoprene and low density polyethylene are particularly susceptible to metal catalyzed oxidation.