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1 Antioxidants & Antidegradants Polymer Degradation Chemistry

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Antioxidants & Antidegradants All polymers & products baded on them are subject to degradation on exposure to the degradative environments 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 rubbers / rubber products causing substantial changes in their technical properties and ultimately lead to their failure during service or shorten the expected service life in the absence of Antioxidants. Initiating / Accelerating Degradation Type of Degradation Type of Failure Factors Causes Oxygen, Light, Storage Aging Surrounding conditions Loss of elasticity and tensile strength Heat, Humidity Aging due to Heat Heat Oxygen Loss of elasticity and tensile strength Light, UV light, heat, Aging due to Light & Formation of crazed surface, loss of humidity, surrounding Oxygen Weathering elasticity and tensile strength conditions Soluble Metal ion Cu, Mn, Fe, Co & Ni Rapid loss in elasticity and tensile (Cu,Mn,Fe) catalyzed Ions as their rubber Oxygen strength. oxidation soluble fatty acid salts Intermittent mechanical Oxygen, Appearance of cracks (cracking Flex-Fatigue cracking strains Ozone, Flaws patterns usually complex) Continuous / Intermittent Extensive cracking at right angles to the Ozone Cracking Ozone strains force causing strain 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. ‘Propagation’ is the second stage when atmospheric oxygen reacts with the free radicals (R ) to form ‘Peroxy’ . ( ROO ) radicals Peroxy radical further reacts with labile Hydrogen atom of the polymer to form unstable hydroperoxides (ROOH). ROO + RH ROOH + R 1 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. 2 . • 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. • Auto-oxidation depends on the relative C – H bond dissociation energy. Strongest Bond Weakest Bond Primary Secondary Tertiary Allyic 98.0 kcal. 94.5 kcal 91.0 kcal 85.0 kcal R – CH – H R CH – H R C – H RCH=CH-CRH-H 2 2 3 • 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 3 . 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 Degradation on Elastomers Elastomers Effect Of Degradation Natural Rubber ( NR ) Scission ( Softens ) Poly Isoprene ( IR ) Scission ( Softens ) Polychloroprene ( CR ) Cross linking & Scission ( Harden ) Styrene Butadiene Rubber ( SBR ) Cross linking & Scission ( Harden ) Acrylo Nitrile Butadiene Rubber ( NBR ) Cross linking ( Harden ) Polybutadiene Rubber ( BR ) Cross linking ( Harden ) Isobutylene Isoprene Rubber ( IIR ) Scission ( Softens ) Ethylene Propylene Rubber ( EPM ) Cross linking & Scission ( Harden ) Ethylene Propylene Ter-polymer ( EPDM ) Cross linking & Scission ( Harden ) Chlorosulfonated Polyethylene ( CSM ) Cross linking ( Harden ) Polyacrylic Rubber ( ACM ) Cross linking ( Harden ) Fluorinated Hydrocarbon Rubbers ( FPM ) Cross linking ( Harden ) Polysulfide Rubbers ( T ) Cross linking ( Harden ) Chloro Isobutylene Isoprene Rubber ( CIIR ) Cross linking ( Harden ) • This model does not account several factors. Other factors to be considered are : o Relative oxidation rates of different polymers, o Presence of amorphous and crystalline zones in polymers, o Bulk properties which limit oxygen permeation, o Various mix of polymer structures,Reactive nature and structure of propagating radicals . R & ROO • Differences in polymer crystallinity (some polymers have both amorphous and crystalline phase (e.g. SBS copolymer). • Free radicals generally migrate to the amorphous phase and the oxidation takes place in this phase. • The presence of metallic ions such as Cu, Mn, Fe, Co cause catalytic peroxide break down that accelerates the initiation of oxidation. • The permeability of oxygen into a polymer is a key factor that will affect the overall oxidation. 4 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 sooner and 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 Heat on NR Vulcanizates: Temperature, °C Combined Oxygen, % Loss of Tensile Strength, % 60 1.2 50 110 0.65 50 110 Nil. (e.g.N2 atmosphere) Negligible 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. 5 EFFECT OF HEAVY METAL IONS: • Hydro peroxides (ROOH) are the main source of free radicals for the initiation of autocatalytic oxidation reactions. • Decomposition of hydro peroxides is accelerated by heat , light and polymer soluble fatty acid salts of metals like Cu , Mn , Fe , Co & Ni . • The rubber soluble metallic compounds catalyze hydro peroxide decomposition to free radicals according to following scheme. • Direct reaction of metallic compound with polymer in the early stage of the degradation may also result in free radicals as follows: . RH + MX2 R + MX + HX RH + MX R . + M + HX • Stearates and Oleates of Cu & Mn are highly active catalysts of oxidative degradation of NR. Less soluble forms like Oxides of Cu & Mn react with fatty acids to produce highly rubber soluble compounds. • The first corrective approach is to eliminate all the sources of these harmful metals or their soluble salts. It is possible to protect polymers by incorporating substances which react with ionic metals to give stable co-ordination complexes. EFFECT OF DYNAMIC FATIGUE: • Vulcanized rubber surface often has a few flaws resulting from several factors such as rough mould surface, grit content in rubber & other compounding ingredients, ozone attack. • During repeated deformation, the stresses get concentrated at these flaws or cracks which grow under repeated dynamic conditions causing mechanical rupture. • Flex-Fatigue cracking is also due to rubber being subjected to repeated deformations in air (ozone may be absent!) resulting in oxidative fatigue break down. • The Antiflexcracking agents, in addition to being Antioxidants and Antiozonants; possess the ability to reduce the rate of crack growth. • Rate of oxidation of rubber is proportional to the amount of Combined Sulphur Atoms in the cross linked net work. • The Semi EV & EV cure systems
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