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. Type of Degradation Storage Aging Initiating / Accelerating Factors Surrounding conditions Degradation Causes Oxygen, Light, Heat, Humidity Type of Failure Loss of elasticity and tensile strength Aging due to Heat Heat Oxygen Loss of elasticity and tensile strength Aging due to Light & Weathering Soluble Metal ion (Cu,Mn,Fe) catalyzed oxidation Flex-Fatigue cracking Ozone Cracking Light, UV light, heat, humidity, surrounding conditions Cu, Mn, Fe, Co & Ni Ions as their rubber soluble fatty acid salts Intermittent mechanical strains Continuous / Intermittent strains Oxygen Oxygen Oxygen, Ozone, Flaws Ozone Formation of crazed surface, loss of elasticity and tensile strength Rapid loss in elasticity and tensile strength. Appearance of cracks (cracking patterns usually complex) Extensive cracking at right angles to the 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 ( ROO. ) radicals (R. ) to form Peroxy 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 2 H R 2 CH H R 3 C H RCH=CH-CRH-H 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 R 2 HCOO. R 2 C=O + R 2 CHOH + O 2 ROO. +. OH ROH + O 2 Scission predominates in polymers like NR, IR, IIR (unsaturated polymers which have electron donating groups such as -CH 3 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 R 3 COO. R 3 COOCR 3 + O 2 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 o o o Relative oxidation rates of different polymers, Presence of amorphous and crystalline zones in polymers, Bulk properties which limit oxygen permeation, 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.n 2 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 + MX 2 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 (with fewer number of combined sulphur atoms per cross link) in NR based compounds exhibit good heat ageing properties but poor dynamic flex-fatigue resistance. 6
However, SBR based Semi EV & EV cure systems exhibit both good heat ageing & dynamic flexfatigue resistance. Ozone cracking and Flex cracking may occur at the same time involving complex mechanisms. Para-phenylenediamine antidegradants (PPDs) offer excellent protection to rubber vulcanizates as Antioxidants, Antiozonants &. Antiflexcracking agent. 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. 7
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. The ozonation reactions proceed as follows : Under strain these ozonides easily decompose and break the double bonds resulting in the appearance of surface cracks and as this mechanism repeats, the cracks grow deeper. Under unstressed conditions, a silver grey film is formed on the rubber surface which is known as frosting. Frosting is accelerated by hot & humid conditions. 8
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 a harmless radical) 9
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, - Hydro peroxide decomposers, - Metal deactivators, - UV 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. 10
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. CLASSIFICATION OF ANTIOXIDANTS: According to Function According to Chemical Types According to ASTM D4676 / Non Phenolic Class 1 Para Phenylenediamines (PPD s) Antioxidants Phosphates Class 2 Trimethyl-dihydroquinolines (TMQs) Antiozonant / Anti flex cracking Agent Thioesters Class 3 Phenolics Cure / Cross link modifiers Amines Class 4 Alkylated Diphenylamines (DPAs) - Multifunctional Class 5 Aromatic Phosphites - - Class 6 Diphenylamine - Ketone Condensates The antioxidants can also be classified in two types according to the way in which they function : - Preventive antioxidants which inhibit the formation of free radicals (R. ) during the initiation stage. - Antioxidants which interrupt the propagation cycle by reacting with (R. ) and (ROO. ) radicals and thus introducing new termination reactions. Hindered Phenols and Secondary Aryl Amines act as Primary Antioxidants by donating their reactive Hydrogen (O-H, N-H ) to the polymer free radicals, particularly Peroxy radicals, formed during the propagation stage of polymer degradation process. ROO. + AH ROOH + A. 11
The antioxidant radical (A. ) formed during the above process must be stable to discontinue the propagation of new radicals. The radical (A.) in most cases is stabilized by electron delocalization or resonance. PARAPHENYLENE DIAMINES ( ASTM : D4676 CLASS-1). The general structure of PPDs can be represented as follows : Three types of PPDs are used in the Rubber Industry. 1. PPD Type I : N, N -dialkyl-p-phenylenediamine. 2. PPD Type II : N-alkyl-N -aryl-p-phenylenediamine. 3. PPD Type III : N,N -diaryl-p-phenylenediamine. Paraphenylene Diamine antidegradants (PPDs) function as primary antioxidants and are recognized as the most powerful class of chemical antiozonants, Antiflexcracking agents and Antioxidants. PPDs are extensively used in tyres, beltings and molded & extruded rubber products as antiozonants & Antiflexcracking agents at 1.0 4.0 phr dosages. However, being highly staining & discoloring type, PPDs are not used in white or colored products. PPD antidegradants are also used as polymer stabilizers. PPDs vary in chemical structure and in performance characteristics depending on the substitutions on the nitrogen atoms.. R & R 1 are secondary alkyl groups (C6 or larger). These PPDs are generally liquids at room temperature and represent single chemical component > 90%. 12
In case of 77PD both the substituent groups are alkyl groups (branched C7). In case of 88PD both the substituent groups are alkyl groups (branched C8). Dialkyl PPDs : - Offer excellent static ozone resistance even in the absence of wax. - Are not very effective under dynamic conditions. - Do not leach out in water. - Are more sensitive to oxygen and hence suffer from lack of persistency and poor shelf-life (3 to 4 months only). - Are highly basic in nature and hence scorchier. - Used in combination with alkyl-aryl PPDs to obtain static and dynamic ozone protection. - Dosage 1.0 to 2.0 phr. 13
R is a secondary alkyl group and R 1 is hydrogen or a primary alkyl substituent (usually methyl). These PPDs are generally consist of a single component or purposeful mixture of two or more major components. The products can be liquids or solids. In case of IPPD, R is isopropyl group and R1 is hydrogen. In case of 6PPD, R is (1,3-dimethylbutyl) group and R1 is hydrogen. In case of CPPD, R is cyclohexyl group and R1 is hydrogen. In case of 8PPD, R is octyl group and R1 is hydrogen. Alkyl Aryl PPDs: - Offer best all-round performance as Antidegradants against all degradative forces. - Offer excellent antioxidant, static & dynamic ozone resistance and anti-flex cracking properties. - Exhibit optimum migration rate due to the presence of bulky aromatic ring on one side and a branched alkyl chain on the other side. - Are less volatile than di-alkyl PPDs. (Lower losses during storage, processing & cure and hence offer long term protection to rubber vulcanizates). - Are slightly basic in nature and hence influence scorch & cure characteristics. - The water leaching properties depend on molecular weight of the - PPD and the ph of water. (e.g. IPPD is easily leached out in water but 6PPD does not leach out in water to any appreciable extent.) - Exhibit high solubility in rubber hence do not exhibit blooming tendencies. - Dosages used in rubber compounds are from 1.0 phr to 4.0 phr. Their effectiveness increases as the dosages are increased. - Are stable & have good shelf-life (12 months). 14
15
R and R 1 can be the same or different groups consisting of hydrogen or alkyl groups (usually methyl). Therefore this type of PPDs can be individual components or mixtures of three or more isomers. Diaryl PPDs are generally solids at ambient temperatures. In case of DPPD both R & R 1 are hydrogen atoms. In case of DTPD, R 1, R2, R3, R4 may be hydrogen or methyl groups. Comparatively slower migration rate due to the presence of bulky aromatic rings and hence persistent, non-extractable by fuels & solvents and is retained in the compound (in which originally incorporated) to offer long term protection. Limited solubility in rubber hence exhibit blooming tendencies ( over 0.7 phr in NR & 1.0 phr in SRs ) Less effective at lower dosages ( but exhibit bloom at higher dosages). Good antioxidant, static & dynamic ozone resistance and anti-flex cracking properties. 16
Comparatively less basic in nature and hence does not influence scorch or cure rate. Generally used in combination with alkyl-aryl PPDs to obtain static and dynamic ozone protection (Dosage 0.5 to 1.0 phr). Resume activity when other PPDs get depleted. More stable & good shelf-life (12 months). Antiozonant Action: Diaryl PPD H N H N O 3 O 2 H H N N O H N N - H 2 O N N O O H O 3 O 2 N N Catalytic process mechanism of polymer protection: How DPPD Antidegradant works. NH NH DPPD NH N. ROO. NH O N ROO. N N R. N N N O N DPQI ROO. O N DPQI M O ROO. DPQIDO O N R POLYMER ATTACHED POLYMER ATTACHED POLYMER ATTACHED 17
PPD antiozonants exhibit following common functions : - All PPDs migrate to the surface of the rubber product to react with ozone directly and competitively. - The migratory losses of antiozonants can occur by migration to adjacent stock. - The effectiveness of PPD antiozonants in rubber vulcanizates can be improved by incorporation of waxes and synergistic antioxidants. This helps in the reduction of PPD dosages for adequate ozone protection. - The use of higher wax dosages / selection of improper wax grades can be detrimental to the performance of Antiozonants. - All PPDs decrease the rate of cut - growth of rubber vulcanizates. - Di alkyl & Alkyl-Aryl PPDs increase polymer s critical stress necessary for ozone crack initiation. - The use of antioxidants along with PPDs protects the PPDs from direct oxidation. - The Dialkyl-PPDs are initially the most active antiozonants followed by Alkyl-Aryl-PPDs & then the Di-Aryl-PPDs. This order of activity reverses as the ageing progresses due to oxidation and exposure of PPDs. - For extended protection of rubber vulcanizates blends of PPD antiozonants are often used. - The solubility of PPD antiozonants depends on the solubility parameter of the rubber itself. (Example: Diaryl PPDs have low solubility in NR and hence bloom above 1.0 phr dosage.) - All PPDs exhibit high solubility in NBR based compounds and hence do not migrate easily to the surface to offer ozone protection. - Diaryl-PPDs are more effective and persistent compared to other PPDs in polychloroprene (CR) based compounds. However, PPD antiozonants can cross link CR or other halogenated polymers causing bin cure. - PPD antiozonants are easily oxidized by oxidizing agents (e.g. Lead Oxide) or even during storage by oxygen or ozone. - PPD antiozonants are not added in the Rubber-Carbon black master batch as oxidation of this blend destroys the activity of PPD antiozonant. The catalytic mechanism generally recognized to occur on inclusion of Phenolic and Amine antioxidants / antidegradants to protect polymeric materials against degradative factors can be represented as follows: 18
In this mechanism, the original diphenylamine antioxidant is converted to diphenylamine-n-oxide. The N-oxide can either trap R. and then thermally eliminate diphenylhydroxylamine directly. The diphenylhydroxylamine then behaves as a hydrogen donating antioxidant by neutralizing ROO. and the catalytic cycle is complete. This process uses polymer as a source of hydrogen to moderate, compete with and control the propagating step of the free radical oxidation process. The creation of double bonds in the polymer has minimal effect on the overall physical properties because polymer chains are not being broken or cross-linked. This mechanism is often called as Chain breaking hydrogen abstractor / Chain breaking hydrogen donor mechanism. COMPARISONS OF ANTIDEGRADANTS IN NR-TRUCK TREAD COMPOUND @ 2.0 phr. Antidegradant M.P., C MS @ 120 C, min NR Protection SR Protection Static Dynamic Static Dynamic Demattia Flex, kc Control - 27 - - - - 87 IPPD 75 23 100 100 100 100 430 6PPD 48 26 100 90 85 80 300 77PD Liquid 19 160 65 130 75 200 DTPD 105 15 150 65 125 70 225 - DTPD is expected to show comparatively much better performance during long term exposure tests due to its slower migration rate. - NR & SR ozone protection under static & dynamic conditions as per standard ASTM procedure indexed @ IPPD = 100. COMPARISON OF TMQ AND 6PPD IN TRUCK TREADS Base Formulation: NR - 75, BR1220-25, N339 Black - 50, Aromatic Oil - 8, Zinc Oxide - 5, Stearic Acid - 2, CBS - 0.6, Sulphur 2.5 Fatigue Life, kc to failure at 100% Extension 6PPD/ TMQ Dynamic Ozone Dosages, phr % T.S. Retention % Improvement over Resistance @ 20 (85 C 12 days) Aged Unaged Control pphm. 96 hr. (85 C-5 days) TMQ 6PPD Unaged Aged (Crack Rating) - - 28.5 30.0 15.0 - - 1 1.0-68.0 50.0 42.0 167 280 4 2.0-88.0 60.0 58.0 200 387 4 3.0-94.0 62.0 60.0 207 400 4-1.0 62.0 132.0 75.0 440 500 7-2.0 89.0 180.0 110.0 600 733 9-3.0 100.0 195.0 125.0 650 833 9 Ozone Crack Rating : 10 No Cracks, 0 Complete Failure 19
The test results indicate that: 6PPD Antidegradant provides high degree of protection against Oxidative Heat Ageing, Flex-Fatigue (Unaged & Aged), and Dynamic Ozone Resistance even at 1.0 phr dosage. The degree of protection increases as the dosage is increased further. The test results confirm that 6PPD Antidegradant is almost three times more effective than TMQ antioxidant in this respect. TMQ antioxidant offers excellent oxidative heat ageing resistance to rubber vulcanizates which is comparable to 6PPD. Considering the costs involved; one would prefer use of TMQ antioxidant so that the 6PPD antioxidant included in the rubber compound is available for protection against other degradative forces. EVALUATION OF TMQ AND 6PPD IN NR/BR -TRUCK TREAD COMPOUND. Base Formulation: NR - 75, BR1220-25, N339 Black - 50, Aromatic Oil - 8, Zinc Oxide - 5, Stearic Acid - 2, CBS - 0.6, Sulphur - 2.5. 6 PPD / TMQ Dosages phr % Tensile Strength 6 PPD TMQ Retention phr phr (Aged, 85 C-2days) Fatigue Life, kc to Failure at 100% Extension Unaged Aged ( 85 C-5 days) % Retention Of Fatigue Life 1.0-60.0 132 135 102 1.0 1.0 72.0 123 137.5 112 1.0 2.0 78.5 118 142 120 1.0 3.0 88.0 113 154 136 2.0-77.5 135 137 101 2.0 1.0 84.5 130 150 115 2.0 2.0 88.5 135 132 98 2.0 3.0 92.0 135 162 120 3.0-88.0 156 158 101 3.0 1.0 92.5 145 150 103 3.0 2.0 96.0 148 161 109 3.0 3.0 96.5 162.5 187.5 115 The test results indicate that: The inclusion of TMQ antioxidant along with 6PPD improves oxidative heat ageing and the extent of improvement is proportional to the dosages of 6PPD and TMQ. The improvements are smaller at higher dosages under the test conditions suggesting acceptable performance at optimized ratio of 6PPD: TMQ at 2: 1. At higher dosages of 6PPD & TMQ further improvements are obtained indicating options available for obtaining still higher performance. 20
IPPD REPLACEMENT BY 6PPD. (REASONS) 6PPD is not a skin sensitizer like IPPD, 6PPD is much less volatile than IPPD (Better retention in rubber compounds), 6PPD is not easily leached out in water like IPPD, 6PPD is not scorchier like IPPD, If the losses of IPPD due to volatility & water leaching are taken into account, then at equal dosages 6PPD shows better performance than IPPD. THE ROLE OF WAXES. Waxes migrate quickly to the surface of rubber vulcanizates to form a physical barrier against ozone attack. A critical of wax bloom is required to form a continuous film for optimum static ozone protection. Solubility and mobility of waxes govern their ability to form a sufficient level of bloom which is also dependent on polymer & filler type added, loadings, state of cure and time & temperature of storage after vulcanization. Paraffin waxes are produced by solvent extraction of lubricating oil fractions and then separated into desired sub-fractions by step wise crystallization. Paraffin waxes contain normal paraffins (alkanes) with slightly branched chain paraffins (iso alkanes) with generic formula (C n H 2n+2 ) where n = 18 to 50. The Melting Points vary from 52 to 54 C (C 18 to C 36 ) to 66 to 68 C (C 22 to C 50). Low molecular weight fractions (< C 24 ) migrate faster at lower temperatures e.g. 0 C but dissolve in rubber as the temperature rises. High molecular weight fractions ( > C 24 ) migrate slower at low temperatures but faster at high temperatures e.g. 40 C A typical Blended Paraffin Wax grade contains a mol. wt range of C 19 to C 48 with majority fractions of C 25 to C 33 range and thus provides a sufficient range of fractions which responds over a wide range of ambient temperatures. The Microcrystalline Waxes (MC-Waxes) are amorphous in nature and are extracted from higher boiling lubricant oil fractions. These are predominantly branched chain saturated structures with C 34 to C 70 chains with Melting Point range of 57 C to 99 C and typical carbon chain length of C 60. Paraffin wax has large and well defined crystals where as MC-Wax has crystals which are small and of irregular shape. The branched nature and higher mol-wt of MC-waxes result in slower diffusion rates through rubber vulcanizates at lower temperatures and are effective when the rubber vulcanizate temperature is high. 21
Paraffin waxes offer best static ozone protection at low temperatures while MC-Waxes protect best at high vulcanizate temperatures. A blend of Paraffin Wax & MC-Wax would provide best protection at ambient exposure temperatures of rubber products under static conditions. No chemical reactions are involved between wax and ozone. Use of waxes in rubber compounds reduces the fatigue life and dynamic ozone resistance of the rubber vulcanizates. Waxes alone do not offer ozone protection under dynamic conditions due to lack of adhesion between wax film & vulcanized rubber surface and the inextensibility of the wax bloom (film). The PPD antidegradants do provide ozone protection against both static and dynamic conditions due to Chemical Reactions with ozone besides film formation. Any change in ozone concentration and atmospheric temperature will influence the rate of chemical reactions between ozone and PPDs. Increase in PPD dosages results in slight increase in wax bloom and thus at lower wax dosages the ozone resistance is improved significantly. The wax bloom thickness is higher in case of NR / BR blends than 100% NR compounds. At normal levels of PPD-Wax blends, there is no synergism between PPD and Waxes under static or dynamic conditions. However, significant synergism is observed under intermittent conditions due to a combination of effective static ozone protection of wax and dynamic ozone protection of PPDs. Blended Wax & PPD Antidegradants combinations will provide excellent ozone protection over a wide range of temperatures under Static, Dynamic as well as Intermittent (alternating static & dynamic) conditions. At high dosages of blended wax ( > 3.0 phr ) the flex-fatigue life of rubber vulcanizates reduces due to ozone attack at the cracks alone resulting in increased crack depth of the exposed ( & unprotected ) rubber surface. This can be prevented by the presence of PPD Antidegradant through its chemical reactions with ozone. WAXES AND THEIR TYPICAL PROPERTIES Properties Paraffin Wax MC-Wax PE Wax PP Wax Avg. Mol-Weight 350 420 490 800 % Normal Paraffins High Low % Iso Paraffins & Napthenes Low High - - Typical Carbon Chain length C 26 C 60 Melting Point, C / Bloom 51-53 / 0.41 74-76 / 1.3 thickness, micron @ 1.5phr 54-56 / 2.4 79-81 / 0.9 85-87 / 0.48 105 / 0.00 (Melting point and bloom thickness have no 56-58 / 3.5 82-84 / 0.22 99-101 / 0.05 direct co-relationship. Values given in this table are typical and after 50 days at Room 59-61 / 2.64 84-86 / 0.15 Temperature). 64-66 / 2.26 89-91 / 0.08 - - 67-69 /2.06-22
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TRIMETHYL-DIHYDROQUINOLINES (TMQs) ( ASTM : D4676 CLASS-2) TMQs represent a group of antidegradants based on polymerized Aniline-Acetone condensation products. The individual products differ by the degree of polymerization and Dimer + Trimer + Tetramer Contents. TMQs function as primary antioxidants and are recognized powerful class of chemical antioxidants, used to protect rubber articles from degradation by atmospheric oxygen at higher temperatures. It is well known that only the secondary aminic groups (= N H) function as antioxidants by the formation of nitroxyl radical ( - NO ) and thus the Dimer, Trimer & higher oligomers content in TMQ only act as effective antioxidants. 24
The Contents of Amine Impurities (which cause several processing problems) also differ considerably from supplier to supplier. The primary aminic groups (- NH 2 ) do not function as antioxidants. Hence products which contain higher proportions of primary aminic groups are weaker antioxidants and are responsible to activate sulphur cross linking and scorching of Rubber Compounds. Primary aminic groups destabilize Insoluble Sulphur by decreasing its transition temperature to soluble Sulphur and thus causing scorching and sulphur bloom problems for Rubber compounds. The formation of isopropyl-bis-aniline (or bisaniline A) and Monomer-Aniline Adducts during TMQ manufacture; are responsible for dramatically decreasing the Peptization activity as well as initiating cross linking reactions causing viscosity increase of NR based compounds at the rubber processing temperatures. The monomer content in TMQ antioxidant is also considered as an objectionable impurity since it is leached out in water and can cause porosity in thicker cross section extrudates. TMQ oligomers are widely used in the Rubber Industry as general purpose, high activity and inexpensive amine class antioxidants and offer excellent resistance to thermo-oxidative ageing of elastomers & their vulcanizates. TMQs are in tyres, beltings and molded & extruded rubber products as powerful antioxidants at 0.5 2.0 phr dosages. TMQs are moderately staining & discoloring type, TMQs can be used in colored products at small dosages. Low volatility of TMQ due to polymeric nature ensures maximum retention in Elastomers. Low mobility of TMQ ensures high retention and minimizing losses through diffusion or extraction. TMQ is Highly persistent & Non-blooming antioxidant with minimal effects on the processing and curing characteristics of Rubber Compounds, Due to Very high activity; TMQ is effective even at lower dosages, TMQ is only mildly discoloring and does not cause any contact staining. 25
MECHANISM OF TMQ ANTIOXIDANT REACTIONS TMQs act as alkyl radical (R. ) scavenger which immediately stops auto-oxidation process. The hindered aromatic amine antioxidants get oxidized due to peroxy radicals to form nitroxyl radicals which act as scavengers of alkyl radicals (R. ). The process proceeds cyclically forming nitroxyl radicals until the nitroxyl radicals are destroyed by side reactions. TMQs Activity in Rubber Compounds: a) Effect of TMQ Dosages in NR Tread Compound (Base Compound: NR-100, Peptizer-0.15, ZnO 5, St. Acid 2, N 330 Black 45, Aromatic Oil 5, Wax 0.5, MBS 0.5, S 2.3.) Ageing in Oxygen Bomb @ 70 C 20 atm pressure. TMQ, phr 0.5 1.0 1.5 2.0 No. of days for 50 % drop in T.S. 10 15 19 21 Even at 0.5 phr dosage, TMQ is an highly active antioxidant for NR. Antioxidant activity improves as TMQ dosage is increased to 2.0 phr. 26
b) Comparison of TMQ against other antioxidants in NR Tread Compound : (Base Compound: NR-100, Peptizer-0.15, ZnO 5, St. Acid 2, N 330 Black 45, Aromatic Oil 5, Wax 0.5, MBS 0.5, S 2.3, Antioxidant 2.5. ) Tensile Strength Retention After Ageing In Hot Air @ 100 C No. of Days Control ADPA (L) TMQ 6 PPD IPPD 1 25.5 50.2 55.3 55.5 55.5 2 10.2 25.5 34.0 38.3 40.8 3 6.8 17.0 21.3 28.1 28.5 4 6.0 10.2 12.8 15.0 17.0 PHENOLIC ANTIOXIDANTS ( ASTM : D4676 CLASS-3) Phenolic antioxidants are primary antioxidants and are classified chemically according to the number of Phenolic groups in the molecule. In general, the more sterically hindered antioxidants are less discoloring but have lower antioxidant activity in rubber application. The Phenolic Antioxidants are Non- type and are used in the Rubber Industry for the manufacture of white/colored rubber products and as stabilizers of raw Synthetic Rubbers. Phenolic Antioxidants are of following types: Mono-functional Phenols (Type 1), Bi-functional Phenols (Type 2), Multi-functional Phenols (Type 3). The Phenol radical (A. ) can cause polymer degradation but this is prevented by the hindered physical structure (e.g. substitution by styrene) at 2,6 position. 27
Sterically Hindered phenols act by scavenging RO. And ROO. Radicals via Hydrogen atom transfer from the OH group to form hydroperoxides and phenoxyl radicals. The reaction mechanism is given below: The radical (A. ) is stabilized by electron delocalization or resonance as shown below: The discoloration tendency of Phenolic Antioxidants is due to the formation of Stilbenequinone which is explained as follows : These products are low cost, weaker, less persistent, exhibit higher volatility & slight discoloration tendency on long term ageing of the rubber vulcanizates. 28
Styrenated Phenol is much less volatile & provides long term protection and is widely used in white / coloured latex based goods at the dosage of 1.0-2.0 phr. BHT is more volatile and provides only short term protection. It is mostly used as an in process stabiliser for synthetic polymers to impart raw polymer storage stability. These represent the most important class of non staining antioxidants. These antioxidants have low volatility, good antioxidant activity and exhibit minimal discoloration to rubber vulcanizates. Depending upon the position of linkages, bisphenols are subdivided into ortho or para bridged bisphenolic antioxidants. The ortho bridged bisphenolic antioxidants exhibit excellent antioxidant performance but show discoloration (pinking) tendency to rubber vulcanizates. The para bridged bis phenolic antioxidants show slightly lower antioxidant performance but do not exhibit discoloration effect on rubber vulcanizates. Thiobisphenols exhibit high antioxidant activity compared to similar bisphenols. Thiobisphenols generate sulphur compounds which react with polymers during antioxidant protection reaction and compliment antioxidant activities of thiobisphenols. Thiobisphenols cause comparatively higher discoloration of rubber vulcanizates than bisphenols. 29
PHENOLIC ANTIOXIDANTS ( ASTM : D4676 CLASS-3) Multi-functional Phenols (Type 3) Multifunctional phenolic antioxidants are very high performance antioxidants, exhibit extremely low volatility and do not cause contact / migratory staining or discoloration of the rubber products. Higher molecular weight of multifunctional phenolic antioxidants also contributes to practically no leaching and extraction from rubber products by water or solvents and ensuring long term protection against oxidation. Chemical structure of multifunctional phenols is given below: Butylated reaction product of para cresol and dicyclopentadiene (e.g. Wingstay L) is the most popular multifunctional phenolic antioxidant used both in solid rubber products and latex based products. The resistance to oxidative degradation of rubber is due to a particularly favorable pattern of substitution on the phenolic group. Color stability is due to the Stearic impossibility to form highly conjugated and colored by-products like quinones Alkylated Diphenyl Amines (DPAs): ( ASTM : D4676 CLASS-4) This class of antioxidants represents Substituted Amine Antioxidants which are complex reaction products of diphenylamine and various alkylating agents. The substituent is selected to achieve a desired balance of cost and performance characteristics. Alkylated Diphenyl Amines are moderately staining and discoloring. These antioxidants are generally used as stabilizers of raw synthetic polymers and as general purpose antioxidants for rubber vulcanizates. 30
Aromatic Phosphites: ( ASTM : D4676 CLASS-5) Aromatic phosphites are phosphphorus esters of aromatic phenols. Diphenylamine Ketone Condensates: ( ASTM : D4676 CLASS-6) These antioxidants are complex reaction products of diphenylamine and alkyl ketones (primarily acetone), some of which are further condensed with formaldehyde to produce products of high molecular weight. These antioxidants are low melting point resins or liquids. 31
There are two different types of ADPA condensates viz. low temperature reaction products and high temperature reaction products. The high temperature reaction products are reactive towards oxygen and provide some flex cracking resistance but are severely staining and discoloring. They are also volatile and do not provide long term protection against the degradative forces that constantly act on rubber products. These products are mostly used in cheap mechanical rubber products, some non critical tyre applications and as a stabilizer for emulsion SBR. These antioxidants reduce the building tack of uncured rubber compounds making product (e.g. tyres, conveyor beltings etc) building process more difficult. The low temperature condensation products are not very reactive towards oxygen but are less staining and discoloring. These are comparatively less volatile and can provide medium term protection to the rubber vulcanizates. These types of ADPAs have low solubility in oil and hence suitable for oil resistant compounds based on NBR. The recommended dosage of ADPA antioxidants is 1.0 2.0 phr for various rubber products manufacture. PROPERTIES OF ANTIOXIDANTS / ANTIDEGRADANTS For selecting a proper Antioxidant/Antidegradant for specific end-use requirements following factors are considered to be important for the choice. a) Discoloration &, b) Volatility, c) Solubility, d) Chemical Stability, e) Physical form, f) Antioxidant/Antidegradant concentration, g) Cost, h) Health & Toxicity concerns. Discoloration & Discoloration refers to colour imparted to the rubber compound itself. takes place in two forms viz. Contact stain and migration stain. Contact stain is a discoloration or stain imparted to another surface which is in contact with rubber. Migration staining is a discoloration or stain which results in an article adjacent to or nearby the rubber compound. 32
All these forms of discoloration are due to oxidation products of the antioxidants themselves in nearly all the cases. In general, phenolic antioxidants are non-discolouring and the amines are discolouring. Volatility Volatility of antioxidant is related to both the molecular weight and the type of molecule. Generally, greater the molecular weight, less is the volatility. The type of molecule however, has a greater effect than the mol. Weight (e.g..hindered Phenols) has high volatility in comparison with amine antioxidants of the approx. same molecular weight. Loss of antioxidants results due to low volatility during processing, curing and usage of the rubber article under severe operating conditions. VOLATALITY OF ANTIOXIDANTS / ANTIOZONANTS AT 100 C Antioxidants / Antiozonants Loss on Heating at 100 C, % w / w 4 Days 8 Days 12 Days ADPA ( Liquid - Low Viscosity ) 37.00 48.00 54.00 ADPA ( Liquid - High Viscosity ) 28.00 35.00 40.00 ADPA ( Solid - Resin ) 0.60 0.50 0.50 77 PD 7.50 13.20 17.00 IPPD 4.25 6.40 9.00 6 PPD 2.50 4.50 6.20 DTPD 2.50 4.00 6.00 PBNA 2.50 4.70 6.30 TMQ 0.15 0.20 0.20 Solubility Solubility of antioxidants in rubber as well as in solvents (in which the rubber article would come in contact with) is an important factor to be considered while making the choice of antioxidant for a particular end-use. Poor solubility in rubber of a particular antioxidant means only small quantities of the antioxidant can be added without causing any bloom. Solubility can also affect the performance, processing, leaching out in water, solvent and even FDA status. Phenolic and Phosphite antioxidants generally have high solubility in rubber and blooming does not occur. 33
Chemical Stability The stability of an antioxidant towards, heat, light, oxygen etc. is important if it is supposed to have maximum effectiveness for long periods. Many amine antioxidants are affected by oxidative reactions and phenolic antioxidants are affected by heat in the presence of acidic materials. Among the PPDs, dialkyl PPDs are oxidised very fast and are effective for short duration whereas alkyl aryl PPDs and diaryl PPDs are more persistent. Physical Form Solid, free-flowing and non-dusty material are generally preferred over liquid antioxidants because of ease of handling & weighing, Health Safety & Environmental Concerns & change over to automated conveying & weighing systems. Antioxidants / Antiozonants Concentration Determining proper level of Antidegradant to use is a complex question whose answer depends on cost, polymer type, end-use, application, staining requirements, etc. Most materials show an optimum level, based on laboratory ageing studies and increased levels are not required. One is encouraged to use sufficient material to ensure the presence of an optimum dosage so that after extended use part of the antioxidant may be destroyed or rendered inactive and still the rubber product is well protected. This optimum level is very difficult to predict since it depends on so many other factors e.g. curing system, nature of polymer, fillers used, etc. TMQ type antioxidants can be added up to 8.0 phr and the performance is directly proportional to the dosage used for long term ageing of the rubber product. The ppd class of antidegradants can be used from 1 to 4.0 phr level. As far as the non-staining antioxidants are concerned; these are effective at 0.5-2.0 phr range and levels above this are not required. Cost Cost of overall Antidegradant system is the most important factor. For specialised products like tyre, tube, V-belts, hose, etc. this may not be a limiting factor. However; for cheaper rubber products, the antioxidants are selected after considering the total service life expected from the product itself. While selecting antioxidant/antidegradants for products coming in contact with food, or are intended for pharmaceutical use, one must confirm the suitability of the antioxidants chosen for a particular end product. 34
Health & Toxicity Concerns Antioxidants / Antidegradants used in rubber compounds intended for food & drugs contact should be FDA / BGA approved. Use of Antioxidants / Antidegradants of known health hazards should be avoided. Information on Health, Safety & Environmental Control is updated continuously. One must consult a responsible chemical manufacturer for the update before selecting the material for intended use or refer to the latest edition of the BRMA Book. The physical form of the antioxidants may vary from supplier to supplier ( e.g. Liquid, Powder, Flakes, Pastilles, Rods etc.). Ease of handling & transportation, clean & dust-free characteristics are the factors to be considered. How to select the antioxidant system? 1. Identify Primary Degradation Factors. - Oxygen, - Heat, - Light, Weathering & Other gases, - Moisture, Steam, - Ozone, - Metal ions, - Flex-Fatigue and other types of Stresses & Strains. 2. Define Service Environments / Requirements. - & Discoloration, - Temperature, - Static / Dynamic / Intermittent, - Life cycle desired, - Toxicological concerns, - Cost. 3. Select appropriate type of Antioxidant & Antidegradant. - / Non-staining - Mono functional / Multifunctional (e.g. PPD Antidegradant) - Blends of Antioxidants and/or Antidegradants Different functions / Same function but difference in reactivity / Same functionality and reactivity. ) - Refer to (easily) available Product Range and select the appropriate products. If necessary, consult the Manufacturer. 35