ANTIOXIDANT

Antioxidants & Antidegradants

Factors Contributing to Polymer Degradation:

• All polymers (saturated / highly unsaturated) are vulnerable to reactive degradation factors such as:

- Storage aging
- Oxygen
- Heat
- UV Light & Weathering
- Catalytic degradation due to the presence of heavy metal Ions (Cu, Mn, Fe etc.)
- Dynamic Flex - Fatigue
- Ozone (Static / Dynamic / Intermittent exposure)

• These factors degrade polymers causing substantial changes in the technical properties of the rubber products and ultimately lead to their failure during service or shorten the expected service life in the absence of Antioxidants.

Polymer Degradation Chemistry

• The formation of free radicals (R.) during polymerization, processing or service of the rubber product is the first stage of polymer degradation and is called as ‘Initiation’ of degradation process.

Heat,
Mechanical Stress,
Light.

R – R--> 2 R.

• ‘ Propagation ’ is the second stage when atmospheric oxygen reacts with the free radicals (R. ) to form ‘Peroxy’ ( ROO. ) radicals :

R.+ O2--> ROO. (where R. represent free radical)
• Peroxy radical further reacts with labile Hydrogen atom of the polymer molecules to form unstable hydro peroxides (ROOH).

ROO. + RH -->ROOH + R.

• Hydro peroxides immediately decompose via hemolytic cleavage to form alkoxy and hydroxyl radicals and further propagate the degradation mechanism.

ROOH -->RO. + .OH

• Propagation stage of degradation process is very rapid compared to the Initiation stage.

• This autocatalytic oxidation reaction progresses until termination takes place by formation of stable products.

• Free radicals R. can undergo following reactions depending on their own relative stability :

- Dimerize (cross link),
- Disproportionate (exchange H. becoming alkane & olefin),
- Abstract H. (chain transfer),
- Cleave (rupture polymer chain),
- Rearrange,
- React with oxygen.

• Dimerization causes polymer hardening while cleavage reduces polymer chain lengths (change in hardness & elastic properties and causing fatigue – crack initiation points.

• Cleavage may also release of gases (resulting in separations).

• Since all the vulcanization ingredients are still present; degradation can take place by continued changes in the state of vulcanization during the rubber product service.

• This causes Reversion or marching modulus due to changes in the nature of the sulphur cross links.

• The Termination Stage reactions progress as follows :

A) Chain Scission
2 R2HCOO . -->R2C=O + R2CHOH + O2
ROO . + . OH -->ROH + O2

• Scission predominates in polymers like NR, IR, IIR (unsaturated polymers which have electron donating groups such as ‘-CH3’ attached to the carbon atom adjacent to the double bond and hence vulnerable).
•
Scission results in the decrease of molecular weight leading to softening of the aged / over cured vulcanizates, reduction in tensile properties etc.

B)
Cross linking ;

2 R . -->R – R
2 R3COO . -->R3COOCR3 + O2
2 RO . -->ROOR
R . + ROO . -->ROOR

• Cross linking predominates in case of polymers like BR, SBR, NBR, CR, etc. which have comparatively less active double bonds or somewhat deactivated double bonds due to the presence of electron-withdrawing groups such as halogens (e.g. CR, Chloro/Bromo Butyl Rubbers). Cross linking results in brittleness, gelation and reduction in elongation of the polymer.

EFFECT OF OXYGEN:

• Only 1– 2 % of combined oxygen is enough to render the rubber product useless.

• Polymer oxidation is a complex process involving many factors -processing conditions (e.g. temperature, shear rate), presence of catalysts of oxidation, compounding formulation design etc.

• Oxidation causes Chain scission and Cross linking resulting in the loss of elastic properties of vulcanizates. Both occur simultaneously - the one which prevails, determines the final product properties.

• The ‘cure system’ selection also influences the ageing resistance of the rubber product. The ‘Conventional Cure’ Systems are more prone to oxidative degradation than the ‘Semi EV’ or ‘EV Cure’ systems.

EFFECT OF HEAT :

• Heat accelerates the process of oxidation and effects of oxidation are observed soonerand are more severe as the temperature increases.

• In case of NR, in the absence of oxygen, more cross links are formed initially, followed by ‘Reversion’ as cross links and polymer chains are broken.

• The oxidative heat ageing causes loss of Tensile Strength, Elongation at Break and overall Elasticity of the rubber vulcanizates.

EFFECT OF UV - LIGHT & WEATHERING:

• UV-light promotes free radical oxidation of the rubber surface which results in the formation of a film of oxidized rubber on the surface of the product (called as Frosting) .

• Heat & Humidity accelerate this process.

• Light colored rubber products are more prone to UV-light attack than the black colored products (as carbon black itself acts as a UV-light absorber).

• UV-light attack is more severe in case of rubber products with a thin cross section.

EFFECT OF OZONE:

• Longer wave length UV light photolyzes Nitrogen Dioxide to yield Oxygen atoms [O.] and Nitrogen Oxide.

• The Oxygen atoms then combine with Oxygen molecules present in the atmosphere to form Ozone.

• In unpolluted areas the ozone concentration is 2 to 5 pphm. In more polluted areas it can reach to 40 to 50 pphm.

• Ozone is also formed in the stratosphere by the action of short wave length UV-light on oxygen. Although the flux of short wave length UV light is absorbed in the upper
atmosphere, the concentration of ozone in the troposphere is still appreciable due to the
presence of Nitrogen Oxide.

• Oxygen atoms liberated by this photolysis of oxygen molecules also combine with oxygen molecules to form ozone.

• Since ozone is formed by photolytic reaction, its concentration in the atmosphere peaks at mid-day and is negligible at night.

• Ozone concentration in the atmosphere is not temperature dependent but it does peak off during the summer months when the sun light is more directly incident.

• The atmospheric ozone concentration varies on daily basis and is dependant on the severity of the sun light, weather conditions, geographic location, air pollution and the season.

• The ozone absorption occurs at a linear rate for a typical elastomer.

• The ozone absorption is proportional to the concentration of ozone.

• The rubber surface which is not stressed also undergoes reaction with ozone to form oxidized film but does not show typical ozone cracks.

• No crack growth occurs unless the specific stress value is exceeded. This value is known as ‘Critical Stress Value’.

• When the rubber is stressed just above the critical stress value, the ozone cracks are few in numbers but are large in length and depth.

• As the stress is increased to a high stress value, the ozone cracks increase in number and are finer in size.

• In addition to large number of double bonds present in the highly unsaturated rubbers, ozone also reacts with saturated polymers and the polysulphide chains at a comparatively slower rate.

• Unsaturated polymers which contain electron donating groups (e.g. methyl groups in NR) are more vulnerable to ozone attack.

• The unsaturated polymers containing electron-withdrawing groups (e.g. Chlorine in CR, Bromine in BIIR) are less vulnerable to ozone attack due to the deactivating effect imposed on the double bonds by the halogen atoms.

• Ozone reacts with the double bonds in the rubber molecule causing chain scission. The chain scission results in the formation of surface cracks in the direction perpendicular to the applied strain.

Role of Antidegradants:

• Antioxidants & Antiozonants are used to protect the polymers from degradation.

• Antioxidants are highly effective ingredients and have a dramatic impact on the service life of the rubber product although being present at extremely low concentrations (0.5 – 3.5 phr).

• Antioxidants do not completely eliminate oxidative degradation, but they substantially inhibit the rate of auto oxidation by interfering with the radical propagation reaction.

• Depending on the types and combinations of antioxidants used, the polymer can be protected during the entire phase of the product’s life cycle.

• The Antioxidants are categorized as :

A) Primary Antioxidants (Chain Terminating)
e.g. Amines & Phenolic.
B) Secondary Antioxidants (Peroxide Decomposers)
e.g. Phosphites & Thioesters.

• Addition of an Antioxidant ( AH ) in small dosage ( 1.0 - 2.0 phr ) interrupts the degradative reactions as follows :

R˙ + AH -->RH + A˙ ……………. (Scavenges free radicals.)
ROO˙ + AH -->ROOH + A˙ …………. (Prevents chain breaking.)
RO˙ + AH -->ROH + A˙ ……........... (Scavenges alkoxy free radicals.)
ROOH + AH -->Inert products ……… (Prevents degradation.)
ROH + AH -->Inert products………. (Prevents degradation.)
• In order to inhibit the degradation cycle (cascading effect) the antioxidants function as follows :

a) Scavenge the free radicals before they have opportunity to grow in numbers rapidly,
b) Reduce the peroxides & hydro peroxides to alcohols before they produce additional radicals.
•Secondary Aryl Amines, Diamines as well as Sterically Hindered Phenolic Antioxidants act by donating their reactive hydrogen atom (N-H, O-H ) to the free radicals as shown below:

R . + AH -->RH + A .
ROO . + AH --> ROOH + A . ………….(A . is harmless radical)
•Thus the Peroxy Radical is offered a more easily abstractable hydrogen by an externally added hydrogen donor (antioxidant) and the polymer backbone remains unaffected until the H-donor (antioxidant) is consumed.

•In the above process, the Antioxidants themselves get converted to relatively stable radicals which do not propagate further.

•According to the mode of action the antioxidants may be grouped as :

H-donors, Hydroperoxide decomposers, Metal deactivators, UV- Light stabilizers etc.

•Amine class of primary antioxidants is highly effective due to their ability to act as chain terminators and peroxide decomposers. Antioxidants of this class are most widely used in rubber compounds requiring high degree of protection.

Role of Antiozonants:

•The exact mechanism of ozone protection of rubber vulcanizates is still not established!

•Following four theoretical models have been proposed :
1. Inert barrier theory.
2. Competitive reaction theory.
3. Reduced critical stress theory.
4. Chain repair theory.

•The Inert Barrier Theory proposes that the antiozonant migrates from the bulk of the rubber to the surface to form a film. This film functions as a physical barrier which protects the reactive polymer double bonds by keeping ozone out of contact.

•The Inert Barrier Theory Mechanism is similar to ozone protection offered by waxes and non reactive polymers such as EPDM, Halogenated butyl rubber, halogenation of the surface of rubber vulcanizate etc.

•The ‘Reduced Critical Stress Theory’ proposes that the rubber vulcanizate surface is modified by the migration of the antiozonant on the surface or just below the surface of the rubber.

•This modification relieves the internal and surface stresses and the vulcanizate behaves as if it was unstressed or at lower than critical stress required for ozone crack formation.

•The ‘Chain Repair Theory’ proposes that antiozonant reacts directly with the ozonide or the carbonyl oxide forming a low molecular weight, inert & self healing film which attaches the antiozonant to the rubber.

•The Competitive Reaction Theory is sub divided into ‘Scavenger Theory’ & ‘Protective Film Theory’.

•The ‘Scavenger Theory’ proposes that as the antiozonant migrates to the surface; it selectively reacts with ozone and protects the polymer double bonds until the antiozonant is exhausted.

•‘Protective Film Theory’ proposes that once the antiozonant has been fully exhausted, the reaction products of the antiozonant form an Inert Protective Film over the surface of the rubber vulcanizate.

•The Competitive Reaction theory is substantiated by experiments and is well accepted.