DEGRADATION DEGRADATION MECANISMMECANISM

•Rubber compounds can be degraded by reactions with oxygen, ozone, light,

metal ions and heat

•Degradation by oxygen and ozone proceeds via different chemical mechanism and results in different effects on physical properties of rubber

• Oxygen degradation results in Oxygen degradation results in hardening (SBR, NBR, CR, EPDM …) hardening (SBR, NBR, CR, EPDM …) or softening (NR, IR, IIR) throughout or softening (NR, IR, IIR) throughout the rubber articlethe rubber article

• Rubbers that do not contain C=C Rubbers that do not contain C=C unsaturation are less sensitive to unsaturation are less sensitive to oxidation than diene rubbersoxidation than diene rubbers

• Ozone degradation results in Ozone degradation results in discoloration and eventual cracking discoloration and eventual cracking of samplesof samples

• Ozone degradation is primarily a Ozone degradation is primarily a surface phenomenonsurface phenomenon

Mechanism of rubber oxidation

InitiationInitiation

RH R* + H*RH R* + H*

R-R 2R*R-R 2R*

R* + OR* + O22 ROO* ROO*

Shear, ΔT

Shear, ΔT

PropagationROO* + RH R* + ROOHROOH RO* + OH*ROOH + RH ROH + R*

+H2O

RO* + RH ROH + R*OH* + RH HOH + R*

TerminationTermination

ROO* + R* ROORROO* + R* ROOR

RO* + R* RORRO* + R* ROR

R* + R* RRR* + R* RR

•The key to the process is the initial formation of a free-radical species.

•At high temperatures and at large shear forces, free radical formation takes place by cleavage of carbon-carbon and carbon-hydrogen bonds

• trace impurities present in the polymer systems could account for the relative

ease of oxidation

•Due to the high reactivity of free radicals, only trace amounts of these peroxides need to be present to provide initiation of the oxidative chain process

The rate of peroxide decomposition and the resultant rate of oxidation is markedly increased by the presence of ions of metals such as iron, copper, manganese, and cobalt

ROOH + FeROOH + Fe2+2+ RO* + Fe RO* + Fe3+ 3+ + OH-+ OH-

ROOH + FeROOH + Fe3+3+ ROO* + Fe ROO* + Fe2+ 2+ + H++ H+

It is important to control and limit the amounts of metal impurities in raw rubber

•Probably an important pro-oxidant for all rubbers is ultraviolet light

Stabilization mechanism of antioxidants

Five general modes of oxidation inhibition are commonly recognized:

- Metal deactivators (EDTA) - Light absorbers (ZnO, TiO2, C.B) - Peroxide decomposers (Mercaptants,

thiophenol , other organic sulfur compounds)

- Free radical chain stoppers - Inhibitor regenerators

• Termination of propagating radicals Termination of propagating radicals during the oxidative chain reaction is during the oxidative chain reaction is believe to be the dominant believe to be the dominant mechanism by which amine and mechanism by which amine and phenolic antioxidant operate.phenolic antioxidant operate.

•The resulting antioxidant radical is more stable than the initial peroxy

• radical and terminates by reaction with another radical in the system

It is important that the level of antioxidant be kept at the optimum, since excess

antioxidant can result in a pro-oxidant effect (A-H + O2 → AOOH).

The secondary antioxidant react with The secondary antioxidant react with the hydroperoxide resulting in an the hydroperoxide resulting in an oxidized antioxidant and an alcoholoxidized antioxidant and an alcohol

•Free radicals develop in a polymer system and react with oxygen yielding a free radical peroxy.

•This free radical, in turn, reacts with a C-H linkage yielding a peroxide and another free radical. This is the “chain reaction” which continues until it is halted.

• It is generally agreed that antioxidant-free radicals are not reactive and are, thus, effective chain stoppers.

Methods of studying the oxidation resistance of

rubber• Accelerated agingAccelerated aging

• DSCDSC

• TGATGA

• Spectroscopic techniquesSpectroscopic techniques

• SEM techniquesSEM techniques

Ozone and Antiozonant Chemistry

Mechanism of ozone attack on elastomers

Ozone reacts with double bond of Ozone reacts with double bond of rubber to form ozonide (I).rubber to form ozonide (I).

• These ozonide cleave as soon as These ozonide cleave as soon as they are formed to give an aldehyde they are formed to give an aldehyde or ketone and a zwitterion (carbonyl or ketone and a zwitterion (carbonyl oxide)oxide)

• Although the cracking of rubbers is related to the reaction of ozone on the double bond, it must be mentioned that ozone reacts also with sulfur crosslinks.

• These reactions however are much slower.

• The reaction of ozone with di- and polysulfides is

at least 50 times slower than the corresponding reaction with olefins.

RSSSR + O3 → SO2 + RSO2-O-SO2R (+ H2O)→ 2 RSO2H